"LITERALLY EVERYTHING"
In the Marvel Universe, the personification of everything is typically rendered as Eternity, a regal and starry, masculine figure. Eternity’s opposite number was Mistress Death, making the two of them an axis representing life and, well, death. Because Eternity also represents space/time, he is depicted with a sister aspect, Infinity. Here, Eternity is time and Infinity is space, while Death is joined by Oblivion, who represents non-existence. These two (or four) abstract entities have an intermediary, Galactus, who stands outside of Life and Death. Other abstracts also exist for different concepts, like the Stranger, Eternity’s children such as Ego and the Seven Friendless, Lord Chaos and Master Order with their intermediary the In-Betweener, Mistress Love and Sire Hate, and more.
However, “everything” is a fungible concept. Eternity represents the entire universe, but Eternity also represents the entire multiverse. Each universe has its own Eternity, who are themselves each a smaller aspect of the Multiversal Eternity. And Eternity and all abstract entities exist within a hierarchy where they must obey the rulings of the Living Tribunal, who has multiversal aspects themselves as arbiter of existence and emissary for the One-Above-All. But if Eternity is “everything,” where do the One's directives come from?
Other locations exist which are outside the universe / multiverse as we know it. The Superflow is 
the space between universes, a conceptual realm navigated by the Builders and the arterial flow of information and ideas themselves. It is the origin point of the White Events which the Builders used to create Starbrands, Nightmasks and other glyph-wielders within a given universe. Exo-Space is the space beyond the universe, the border realm just outside Eternity’s field of existence. It contains the Neutral Zone where matter and anti-matter co-exist in mutual balance. This Distortion Area also serves as the transit point between the normal positive matter universe and the Negative Zone, a realm comprised entirely of anti-matter. (Some figures like Annihilus claim the Negative Zone and the positive matter universe occupy the same "space", meaning the larger our universe expands, the less space exists for the Negative Zone. Annihilus is notably crazy and paranoid, though, so this hasn't been fully confirmed.)
The universe contains a sub-atomic realm or realms known as the Microverse. At one point, 
multiple “microverses” existed, separated from one another by the Space Wall, but later merged into a single cosmos. Although the Micronauts and related beings sometimes call the dimensional space of Earth the “Macroverse,” there is actually a higher plane called the Macroverse which the FF briefly visited, making the Earthly plane a median realm. Below the Microverse is a realm called Underspace and it has a twin realm if one enlarges enough, called Overspace. These realms have not been explored thoroughly – the Avengers Infinite Mansion once occupied Underspace, and Hank Pym visited Overspace where the Abstract Entities seemingly meet to confer with one another outside their own realm. Overspace may therefore be the same as the Dimension of Manifestations, a similar space identified as a gathering point for the abstracts. Overspace is also the location where Dominions congregate, the highest level of universal-level societies forged of pure condensed data. It’s unclear if Under- and Overspace qualify as part of Exo-Space or rest beyond it. Beyond Exo-Space lies a realm or realms known as the Far Shores, or simply Outside, or Beyond. Understanding this space requires a history of the Eight iterations of the Cosmos thus far.
At the beginning of the beginning was the First Cosmos, also known as the First Firmament. 
Eternity’s eldest sibling or ancestor, the sentience of the First Firmament was an infinite singularity, possessing no multiverse, no concept of time or change. The Lonely One would create life to worship him and these creations in turn created life as well. Eventually, some of his creations rebelled against the monotony and sought to create change. These rebels became what we know today as the Celestials. The Firmament’s loyalists, called the Aspirants, fought the Celestials until the Celestial War damaged the fabric of the firmament enough that the universe died and was reborn into something new, thereby giving birth to the very concept of “new.” (Though the Aspirants came first, we are more familiar with the Celestials and so they are occasionally referred to in relation as “Death Celestials” or the “Celestial Destructor” in stories.)
The Second Cosmos was the first multiverse, but also the shortest. This sentience experimented with infinite possibilities, anything and everything all at once until it decided to experience dying as well. The Celestials continued their own experiments during the Second Cosmos, and here they were responsible for creating the Beyonders. The Beyonders received infinite power from the Omega Force, originating from this cosmos. The Celestials deliberately made the Beyonders strong enough to destroy even them (or the Aspirants) in case of another war. The Beyonders served as maintenance workers for this new multiverse, helping to guide and direct the unstable soup of chaos caused by infinite possibility in action. They would continue this role even into subsequent multiverses. (Technically, the Celestials created Cal-Horra, who created the Beyonders, but when the Beyonders supplanted the Lost One, they omitted him from future tales of their origin.)
Several other facets of existence also date back to the Beyonders. The Omega Force is connected to the Enigma Force, or Uni-Power, which empowers the beings known as Captain Universe. The Enigma Force purportedly powers the Beyonders’ devices called Concordance Engines and seems to be derived from the White Hot Room. The Phoenix Force, cosmic entity of life and rebirth, is the first spark derived from the power and substance of the White Hot Room. From another metaphorical perspective, the Phoenix is the fire inside the cave that gives light and life to mankind. The Tiger God is then the threat mankind fears outside the cave, in the darkness beyond the light. The full interplay between these different forces is unknown. Finally, the Beyonders are known as the Ivory Kings or Kings in White to distinguish them from the King in Black. The Ivory Kings exist Beyond the multiverse, monitoring it from outside, while the King in Black gets “messy” by inhabiting an engine room within the multiverse. The singular, energy-based symbiosis of the Enigma Force and Captain Universe is mirrored by the endless organic symbiotes like Venom, which are derived from the King in Black.
The Third Cosmos introduced the concept of duality. Existence vs. non-existence, light vs. dark, 
creation vs. destruction, good vs. evil… these originated here. The Third Cosmos was represented by a constant power struggle between Lifebringer One, the first hero, and the draconic living abyss of the Anti-All. One grew stronger at the expense of the other, and vice versa, until Lifebringer One finally defeated the Anti-All and shattered it. It is strongly implied that the dual power of the Sentry and the Void is inherited from the Third Cosmos’s primal forces. Other beings of proemial darkness who emerged in later cosmoses also may have originated from fragments of the Anti-All, such as Knull, Nyx, Amatsu-Mikaboshi, or the Dragon of the Moon.
The Fourth Cosmos introduced the concept of narrative. Beings of this universe were little more than archetypes, a cosmos as written by ChatGPT, but they had motivations, conflict and the basic structure of story guiding them. The earliest incarnation of what we would recognize as Galactus, Devourer of Worlds, appeared in the Fourth Cosmos. Known as What-Must-Be, this destructive narrative represented the inevitable end of stories and may have been one of the Anti-All fragments washing up on the shores of this new multiverse. The Fourth Cosmos ended, but its sentience became an eternal voyager journeying into mystery, sometimes returning to later multiverses as the Queen of Nevers. They served as the embodiment of possibility, the source of what might be rather than what is.
The Fifth Cosmos is where magic became part of the universe. The esoteric arts were primal and unregulated in this era, a means of manipulating reality itself ungoverned by spellcraft, incantations, and other stopgaps from the future. The most well-known entity of the day was Moridun, the equivalent of a Sorcerer Supreme. Aspects of Moridun may have passed down to later multiverses – the pattern of his five eyes could be seen as the precursor to all pentagrams used in magic, and his Lovecraftian appearance may have inspired future demons and Old Ones, such as Chthulu, Shuma-Gorath and more.
The Sixth Cosmos invented science, rules which governed the fundamental forces of nature in 
this universe. The existence of science also helped define and tame magic, with each practice now engaged in its own sphere. The Sixth Cosmos created “the Junction to Everywhere,” which seems to be the same as Overspace. This was the universe of Galan of Taa, the scienceer who one day would become Galactus. The concept of What-Must-Be is known to have re-emerged in the Fifth Cosmos. Moridun survived the destruction of his universe by merging with it, becoming Omnimax the Devourer in the Sixth.
The Seventh Cosmos was the original Marvel Universe, seen in 1938-2015, leading up the events of Secret Wars. Galan of Taa survived the Sixth Cosmos to become Galactus, Devourer of Worlds, in the Seventh. The Beyonders arranged for the destruction of the multiverse as an attempt to prevent the threat of Enigma from rising, which ultimately proved unsuccessful. The power and events orchestrated by Doctor Doom, Molecule Man and finally Reed Richards which restored the multiverse rebooted it so that the Eighth Cosmos continued the events of the Seventh, rather than starting over from scratch.
The Eighth Cosmos is the current version of the Marvel Universe. Certain aspects of the cosmic hierarchy are under review as a result of the “newness” of the universe and its unusual reboot. The G.O.D.S. series has proposed an entirely new hierarchy compared to the Seventh, a side effect of this reality being created / restored by a scientist like Reed Richards. Now there are eight greater gods known as “Ur divisors” who rest on axis of symmetry. Eternity and Infinity represent 
time and space, Chaos and Order remain opposites, but now Oblivion and the Living Tribunal rest on an axis of judgment and finality, and a new axis exists for the Powers-That-Be and the Natural-Order-Of-Things, universal abstracts representing magic and science. Lesser gods exist as a balance for the divisors, such as the In-Betweener and now the Preordained, who sits between the Tribunal and Oblivion, but the Powers and the Natural Order have no intermediary. Life and Death exist as opposing conceptual beings, seen in Captain America, but are not considered part of these greater gods, while the classic Mistress Death has been reimagined as part of the new Infinity Stone of Death. Aspects upon aspects…
The First Firmament attempted to return and supplant Eternity at the onset of the Eighth Cosmos, revealing that it had survived the end of its own cosmos. Indeed, the individual sentiences of all seven prior cosmos still exist (with Infinity and Eternity playing the dual roles of Seventh and Eighth). These Ultimate Ultimates teamed up to stop the First Firmament’s plot and cage it once more. The return of the sentiences led to a discussion about what truly lies outside the multiverse, and beyond the border of Exo-Space.
The original title of the Beyonder was the “One From Beyond,” and Beyond is a physical location outside the multiverse where their Engine Room exists. Falling out of the multiverse led Anti-Man 
to Outside, another position to observe Eternity from remotely. The prior Cosmos sentiences emerged from the Far Shores, which is also where the Krakoan Six traveled to mine for Mysterium. The Fourth Cosmos reportedly journeyed into the Mystery, a physical location beyond the cosmos as well. It’s unclear if the Far Shore, Outside, Beyond and the Mystery are different names for the same region of space, or if one is an overall region and the others are specific points within it. A Dominion once identified Beyond / Outside / Mystery as analogous, with Overspace being a subsector of the above, while the Far Shores seem to be the “edge” of the Mystery.
The One-Above-All is generally recognized as the highest possible known figure in the Marvel Universe. The term refers to both the Living Tribunal’s superior, who gave him authority over Eternity and the Abstracts, and is also used for any manifestations that match with the traditional 
Judeo-Christian understanding of God. On a meta-fictional level, the realm of the One-Above-All exists within the Mystery and is referred to as Heaven, or as the House of Ideas, where the One has appeared as an artist resembling Jack Kirby. Primal Kirbon particles from the Mystery are the source for Mysterium, cosmic rays, Pym particles and other amazing and uncanny features of the universe. The One also contains his own opposite, the One-Below-All, whose realm called the Below-Place is representative of Hell. Gamma radiation is uniquely tied to the Below, and the Green Door found in the Below-Place allows gamma mutates to return from the dead.
The Ninth Cosmos has been discussed in abstract on different occasions. Franklin Richards is reportedly destined to replace Galactus as the new Devourer when the cycle begins again. Mr. Immortal is apparently immortal enough that he will survive to the end of the universe as a possible inheritor as well. One version of the Ninth Cosmos showed the Hulk inherited the power of cosmic destruction for the next universe, or Eddie Brock as the multiversal King in Black in the next cycle. A cosmic abstract known as the Griever At the End of All Things represents inevitability or entropy, for all things must end eventually. Another cosmic trinity is suggested to contain Eternity (what is), the Never-Queen (what might be) and the Griever (what must be).
One last aspect of the cosmic architecture is the Infinity Stones. Formerly known as the Infinity Gems, these items can together control all aspects of Space, Time, Power, Mind, Soul, and Reality. According to earlier sources, the Gems were the remnants of the previous sentient cosmos prior to Eternity, who ended their own existence and left behind these aspects of their might. That origin has not been directly revisited since the revelations of the hierarchy of cosmoses from the First Firmament to the present Eighth Cosmos. Therefore, questions remain whether new 
Infinity Gems / Stones are created with the ending of each cosmos (i.e. are the Gems in the Seventh Cosmos the remnants of the Sixth, in the Eighth remnants of the Seventh, etc.), or if the Infinity Gems were the unique remnant of a specific cosmos. The Second Cosmos is the only one explicitly known to have ended its own life, but the Stones have also been tied to the God Quarry, the region which separates the current cosmos from the entropic space previously occupied by the First Firmament. During the Seventh Cosmos in the Ultraverse, a seventh Ego Gem was introduced that bound the gems together as a cosmic being called Nemesis. In the Eighth Cosmos, Mistress Death's incarnation now existed as an Infinity Stone of Death. The secrets of the Gems, or the Stones, remain unexplored.