People start games, they have lore, maps, but how does it work as a whole?
Welcome! I'm honestly not sure how to answer in general, but I'd start out by saying that there's a whole slew of different types of MGs, so how they work tends to vary from game to game.
Any specifics? is there a page with a list of them?
There's the SW page on the wiki, though that doesn't usually describe the game rules. A good place to look would prolly' be the first page of a lot of games. For example, Historiae Mutetur has their rules on the first page.
There's always the ISOT/new map games planning threads as well, though those are mainly for discussion of ideas.
Where can i find the planning threads and such?
Here's the ISOT planning thread (it's mainly used to plan out series of games like FISOT or individual games like AKISOT or PISOT) https://www.alternatehistory.com/discussion/showthread.php?t=240871
And here's the general new games/ideas thread. https://www.alternatehistory.com/discussion/showthread.php?t=156309
Also, there's the Countries of an Alternate World thread (basically coming up with a PoD and then filling a blank map with nations based on the PoD). https://www.alternatehistory.com/discussion/showthread.php?t=313270