What was established, by which entry, and which of the things people say about it the material actually supports.
The multiverse is the load-bearing idea of everything made since 2021, and it is explained across several entries rather than in one. Here is what has actually been established, and where.
There was one timeline, and it was maintained
For thirteen years this story had a single history. Loki reveals that this was not natural: an organisation outside time had been detecting every branch and deleting it, which is why there had only ever been one.
That organisation is the Time Variance Authority, and the crucial fact about it is that the protection was a choice somebody made, not a law of physics.
It stopped being maintained on purpose
The first season of Loki ends with that protection deliberately ended. Every multiverse story afterwards is downstream of a decision taken in one room by two people.
Crossing between universes is possible and expensive
Spider-Man: No Way Home establishes that a spell can pull people across, badly. Doctor Strange in the Multiverse of Madness establishes dream-walking, and that one universe colliding with another destroys both. Deadpool & Wolverine establishes that an organisation exists to manage the consequences.
What the material does not say
It does not say that every version of everybody exists. It does not say that anything you can imagine is out there somewhere. And it does not treat the other universes as interchangeable: the ones we visit are specific places with designations, and this catalogue records those designations on the entries that name them.
Show detailsWhy some entries are flagged and others are not
An entry is marked as an alternate universe when its own sources say it happens somewhere other than the main timeline. That is why What If…? carries the flag and Eyes of Wakanda, which is animated too, does not.
The flag follows the source rather than the medium or the studio.