you are toally wrong
Is it not possible (or allowed) to have any FB permissions at all in a IG. You cannot have username, email or anything like that.
But the firstname of the players is basically enough to see in an multiplayer game so, and that is provided in IG.
No (realtime) multiplayer support exist at all in IG sdk, so you have to use a 3rd party for that. Like Photon, Firebase, PlayFab or similar. Depends on what you need.
But when you say you are new to FB games, just try to create an easy one to begin with.. just a tip