Google Play User Authentication

Hi all! Not sure if it is in the correct section among the almost 80 to choose from:
Can anyone share his solution to Authenticate in Google Play Services for Android projects?
I paid the Google 25$ fee, followed tutorials for days, and installed the Unity plugin GitHub - playgameservices/play-games-plugin-for-unity: Google Play Games plugin for Unity
I did not find an official Unity guide apart from this link, but it did not work for me. I read in this forum the same issue from other users and the reply from Unity staff, but it did not clarify the process.
So, I followed religiously YouTube videos about Google Play Authentication in Unity but they did not work.
I feel overwhelmed with what I think should be an easy solution for an engine specialized in Android projects, and the only bullet I have is that anyone can share his process to authenticate users through Google Play in Apps to store their info in their Google Account.
I hope someone can help