How the multiverse works in the Marvel movies

gamesradar.com:

Doctor Strange 2 goes all-in on the multiverse – exactly what you'd expect from a movie titled Doctor Strange and the Multiverse of Madness. But how does it all work? 

We've already had a taste of the multiverse in Loki and Spider-Man: No Way Home – Loki saw the titular trickster come face to face with several variants (i.e. people that diverge from a selected 'main' timeline and create another timeline with different events and history), while No Way Home saw Tom Holland's Peter Parker come face to face with Tobey Maguire and Andrew Garfield's versions of the character after a spell cast by Doctor Strange went wrong. 

So, let's get into it and answer some of those questions. Buckle up, we're heading into the multiverse. Spoilers for Doctor Strange and the Multiverse of Madness lie ahead, too. 

The multiverse is a collection of alternate universes. 'Our' universe, or the universe where the majority of MCU movies and shows take place, is called Earth-616 – each universe has a designated number to tell them apart. As well as taking place on Earth-616, a significant portion of Doctor Strange 2 is also spent on Earth-838. And if you're wondering why 616 in Doctor Strange 2 got cheers in the cinema, it's because the number has a comic book past.

As for how Doctor Strange is able to travel between universes in the sequel, that's thanks to the powers of America Chavez. America has the ability to open portals to other universes, but she struggles to control her power and use it on command. 

Universe-hopping comes at a price, though – when two universes collide, an incursion occurs. This results in both universes being destroyed and the multiverse contracting. According to comic book lore, each time a new incursion happens, the