This is a good explanation of the feature for use with Lemmy.
I’m gonna be an annoying pedant for a second though and say that “hide my email” and “sign in with apple” are two different and unique features, though they both act as an email relay.
The former just creates a semi-random email address for you that forwards to your email. This address will end in “@icloud.com,” making it indistinguishable from any other iCloud email address (other than the ridiculous address you get).
The latter is an authentication system that allows you to sign up for services that support it with your apple account. The service doesn’t get your password directly of course, when you click the “sign in with apple” button it will redirect you to an apple sign in, typically faceID or touchID, and when you sign in successfully (or are already signed in to Apple) it will redirect back to the service with a token that says “this person is cool.”
Importantly for this conversation though, you can optionally send the service your real email or a generated email address as a relay, but the generated email is not the same as the email generated by “Hide my email,” instead it’s a clearly random series of characters and ends in “@privaterelay.appleid.com”.
In the end they have roughly the same purpose for your email, but the important bit here is that on Lemmy, only “hide my email” will be useful. There are no Lemmy instances that support “sign in with Apple” (yet).
Okay, I’ll stop being a pedant now. Sorry for being annoying there.