I could write an entire blog post about how you can use Shin-Nil's Godot Admob Plugin to implement Google Admob into your Android game made with the Godot Engine...
OR...
I could just link the video I already made regarding the topic-
Quick outline of the process-
Prerequisites-
List app in compatible playstore
Set up Google Admob account*
Wait for app store listing to be approved*
1. Install android custom build template
3. Open the android folder within your project folder("res://android", but your local path)
4. Modify AndriodManifest.xml to include-
"<meta-data android:name="com.google.android.gms.ads.APPLICATION_ID"
android:value="ca-app-pub-xxxxxxxxxxxxxxxx~xxxxxxxxxx" />"
[replace with your app's id]
5. Extract contents of admob-plugin (GodotAdmob.gdap and GodotAdMobX.X.X.release.aar [x.x.x being the version you downloaded]) to "res://android/plugins"
6. Extract entire admob-lib folder to your project root directory ("res://")
7. Open your project
8. Add admob node to a scene ONLY ONCE. Ensure its on a node that is always being used when you want your ad to be displayed
(Requires completed app store listing and admob account creation)
9. Create the desired ad in admob account
10. Enter ad information into admob
11. Go to project -> Export -> Android -> and enable "custom build template", "Godot Admob plug in", and allow permission "access network state".
Test it! See the video for more detailed information.