Well, if you want a explanation from the comics, this reddit thread actually goes into pretty decent detail:
www.reddit.com/r/AskScienceFiction/comments/mlyucu/marvel_do_the_universe_numbers_have_any_cosmic/
Doctor Strange in the Multiverse of Madness once again has fans asking about the Marvel 616 and Earth numbering in the Marvel Universe.
nerdist.com
TLDR? In the real world, some comic creator's were taking a jab at Marvel and DC's self-importance. So, they gave the Marvel Universe a number that didn't seem all that special, at least at first glance.
In fictional terms...it gets retconned a lot. There are actually a LOT of people who COULD be the ones giving out multiversal numbers: The Watchers, The Captain Britian Corps (these guys are the most likely source of the numbers themselves...usually...probably), The Time Variance Authority, The Timebreakers from Exiles (although, it was retconned that since the Crystal Palace moves along with the multiverse it retcons it's own history to suit wherever it is in the multiverse at the time, so the Timebreakers may only be the protectors of the multiverse sometimes), The Inheritors and The Web of Life and Destiny from Spider-Man...the list goes on.
Mostly, the numbers are random, or are some sort of joke or reference to the date the universe was created in the real world.
But, if you want, you could think of them like radio stations, if that helps. A radio station's number isn't really based on anything but what frequency it's broadcasting on, much like how different universes "vibrate" at different frequencies from each other. (...I don't know anything about radio broadcasting, so I might be completely off on that).
We'll see what the MCU does. They'll likely put their own spin on it.
Also worth noting, the Cinematic Universe was Earth-199999...or 616 upside-down (sorta)
621 appearance(s) of Earth-199999 3 appearance(s) in handbook(s) of Earth-199999 22 minor appearance(s) of Earth-199999 18 mention(s) of Earth-199999 1 mention(s) in handbook(s) of Earth-199999 6834 image(s) of Earth-199999 1523 characters that originate from Earth-199999 48 teams that originate...
marvel.fandom.com