Unity plugin indeed totally available, I MADE IT MYSELF GitHub - Thaina/google-signin-unity: Google Sign-In API plugin for Unity game engine. Works with Android and iOS. and I have made a pull request so people can use it from google officially. Google just don’t care to update anything
Same go with play games signin. I ALSO MADE A REPO that make it installable with upm so that it not conflict with jar resolver AGAIN BY MYSELF GitHub - Thaina/play-games-plugin-for-unity: Google Play Games plugin for Unity
So I know everything and we now trying to request unity to make google notice the problem or have unity make a better solution than me. That’s what we are now discussing