Juli Clover explains how Sign in with Apple works and how to use it.
In an app that supports Sign in with Apple, there’s a “Continue with Apple” option that you’ll see when asked to create an account… Apps and websites get no personally identifiable information from you. When you use the feature, apps receive a unique identifier that is distinct for each developer, so there’s no cross-platform or cross-app tracking available.
Apple requires it with iOS and Mac apps. Interestingly, I have yet to see it show up. I suspect it’s not retroactive to apps we’ve already signed into, right?