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.
You are looking for an OAUTH API from any of those providers, if I am understanding your post correctly.
You would read the documentation at their relative websites and then apply what is necessary to your project.
More info on facebook here: https://developers.facebook.com/docs/reference/dialogs/oauth