D&D 5e games with 500 + players

Forgive my ignorance.  I'm used to playing with 5 people sat around a table all working towards the same ends (more or less).  How do the games that have huge numbers of open slots actually work? I've tried to find info online, but short of joining one I can't find anything.  If you connect to the game and there's some 40+ people playing, practically, what happens?  There must be some order to the potential chaos of having characters spread all over the in game world, doing different things.

They are usually one of two things. A living world game or a community for the players to socialize and form groups to play games in. 

If you've seen a campaign with 500+players noted for it, it's likely a gaming hub used for organizing games around a shared Game System/Setting, such as DnD5E. I'm myself member of a "game" with 200+ "players" which is essentially just a fairly inactive hub for a smaller game system. Another possibility is that only a party of more or less normal size plays at any given moment, and it could be a " West Marshes "-style game with a larger casual player-pool from where games are organized. It can also be that one or several DMs runs multiple parallel games in the same campaign out of convenience or shared campaign game. In the end, any game you are applying for should say what their play structure is, and if they somehow aim for a oversized party size or just generally try have way more than 7-8 people online at the same time, they are bound to have some system to organize such things. It's extremely unlikely anyone would play a dnd game with much more than 8 players online at the same time.

The one I’m a GM for exists for the forums, not for the game. Occasionally I launch the game and clean up the splash page. 

Cool, thanks all.  So, basically, joining one isn't like joining a game, and is more like joining a small community who play in the same setting.  Probably better odds of finding an actual game by being in one then.

No problem. We are all glad to help. I play host to a community on Roll20 with over 700 members. The community exists to help players find and play specific types of game style along with answering questions as needed.