Where can I find more information on how to sync up accounts to an APP?

When I say that, I mean if I'm creating an APP that I want to work in conjunction with say instagram, twitch, or facebook, etc, where they log in with their account first, how could I go about that? I'd want to essentially do an ADD-ON and make it more accessible/easy for users. Where could I learn more about how to do that?

It sounds a big vague I know, but I'm not sure how else to explain it.