For the Flutter App, is it possible to configure it to use Sign-in-with-apple using this plugin: https://pub.dev/packages/sign_in_with_apple The documentation mentions Google support but agaik, if an app offers Google login it must also offer Apple login to be approved by AppStore.
Flutter bridge to initiate Sign in with Apple (on iOS, macOS, and Android). Includes support for keychain entries as well as signing in with an Apple ID.