Tere is a two plugins for Unity from Google for having Ads in your app.
First, based on firebase and provided via google play services:
https://github.com/googleads/googleads-mobile-unity
Second one, also well-updated, used by some people plugin for similar purposes as as well as first plugin.
https://github.com/unity-plugins/Unity-Admob
I am new in Ads in Unity3d, and I want to make it clear, what is the difference between them ?
I think someone can give a proper answer.