From what I understood, when the Social API is used for IOs don’t needs plugin to implement Social API’s interfaces, because it uses the default implementation of Game Center.
So my question is for Android. Does it need a plugin to implement or it uses a default implementation like Google Play Service?
Looks like it really needs.
There are steps required to be completed in order to integrate it.
You can read it in the readme GitHub - playgameservices/play-games-plugin-for-unity: Google Play Games plugin for Unity
Also, if you plan to use achievements, make sure you have at least 5 of them. Otherwise Google Play Services might just fail silently.