While testing Admob integration in my mobile apps I was looking for some test ids. As I heard that it is very risky to use actual Ad ids while our app is in the development phase.
So after a little bit search I found a couple of Admob test ids we can use in our test application.
Banner: ca-app-pub-3940256099942544/6300978111
Interstitial : ca-app-pub-3940256099942544/1033173712
Reward Video: ca-app-pub-3940256099942544/5224354917
The value of the admob_app_id field doesn't matter with these Ads. You can use any app id. If your code is correct Ad will be shown in your test app.