Allowing users to register to your app using another service (e.g. Facebook, Spotify, Google) requires OAuth. That’s what I use authlib for, anyways. It simplifies the OAuth process so much that I don’t have to think about the authentication process when designing an app.
I found a short discussion[2] you may find useful.
[1]: https://www.etched.com/
[2]: https://www.lesswrong.com/posts/qhpB9NjcCHjdNDsMG/new-fast-t...