What the Heck is Sign In with Apple?
TL;DR: Basically Sign in with Apple is OAuth, same as Facebook and Twitter, but instead of returning any personal data, Sign in with Apple only returns a sub(ject) string, which is the only way to identify a user. Additional data should be explicitly added by the user within the app (Facebook and Twitter for example, provide app-developers with email address and user names). Facebook might even return waaay more.