Sign in with Apple, introduced at WWDC, isn't as documented as many of us would like. Curtis found this out when he started looking at the new framework for his app, Slopes. For instance, there are several things you need to do once your user has actually signed in.
Apple gives us everything we need to verify users at the time of log in and registration, and it is up to our apps to worry about how we continue to trust them after that, without Apple's credentials.
So They've Signed in with Apple, Now What? by Curtis Herbert