"For example, Apple vaguely states that you can implement Sign in with Apple on Android, but there is no direct documentation on how to do it. We understand that Apple probably doesn’t care much for Android, but if they are going to provide a login system, and are going to force developers of multi-platform apps to adopt it, then providing no real support for a major platform that these multi-platform apps run on is not acceptable."
Eventually you might realize it’s based on an open standard https://openid.net/2019/09/30/apple-successfully-implements-... and that it’s relatively similar to other such standards, except with the option to mask your email, etc.
As an geeky end user, the only way I trust these services for login is if I can link more than one, or even more than one email from the same provider. That way I know I’ll have a backup in case I lose access to the social network or email address that I signed in with... it’s annoying when I can’t add a password or set an email just because I also want to login without a password sometimes...
It's still incomplete, their implementation deviates from the standard or use some lesser used mechanism like the form_post response_type, requiring custom code.
Implementing this was not a pleasant experience.
On the Getting Started  page it lists three options: Apple platforms [use AuthenticationServices], Unity [use the asset from the Unity Asset Store], and "Web and Other Platforms" [use Apple JS/REST]. That "Web and Other Platforms" link provides a wealth of useful documentation .
Tbf, the exact word "Android" is missing, but this is an elementary school-level process of elimination that maybe Android is inferred in the words "other platforms".
So there would be no apple account registered on the phone.
So each app wanting to implement apple login would have to :
- pretty much implement it from scratch
- still have a very subpar experience compared to any other login mechanism (even way worse than email + password) since they would have to ask users to find their obfuscated apple email address.
Still pretty meh that it is the only solution of its kind without an sdk
Any OpenID Connect (or OAUTH2) library of the devs choice is the SDK.
The Facebook/google SDKs are simply there to add trackers and bloat.