Sign in with Apple allows you to use your Apple ID to sign in to Ginger.
Apple maintains a special subscriber email domain - https://privaterelay.appleid.com/
This email address is linked to your Apple ID through their Private Email Relay Service.
If you registered via Apple and opted for this service your account will be associated with this email address. All your payment updates and account associated emails will be sent to that address and be forwarded to your real email address (Apple ID).