Google login, how do you do it

As stated here https://forum.unity.com/threads/google-login-is-still-a-pain-to-implement-in-2021.1134436/ I also find Google login a pain and have not yet succeeded to get this to work in a simple way. I had no problem with Facebook.

The way I would like to do it is something like this using a REST client:

. This particular example works for me but it is based on copy/paste, which is not convenient. I have tried to implement the Loopback method so the player do not need to manually copy sign in code but have not been able to get it to work yet and have found no examples on how to do it.

I am considering PlayFab and Google but would like, if possible, to keep this game outside of those provider. However, I will use them if needed.

So my simple question is what method people use to authenticate via Google for both IOS and Android.