Instructions for external or integrating auth?

I’m looking to either use my own auth or use Craft’s for additional functionality outside of Craft. Our client has some very specific requirements that we’ll be building separately and we’d like to save them having to log into the CMS for one thing and a separate system for another.

Are there instructions, docs, tutorials, etc for something like this? Or is it possible?

I’ve seen some plugins for adding things like oauth, but I’m wondering if there’s any official info instead of looking at someone else’s code.