cordova-admob-free plugin as a bridge to call and received AdMob APIs for access to mobile ad networks. And you need class implements for forwards the events to the JavaScript layer.
For example I can't speak English and you can speak English then how can we communicate? to be able to understand each other, we need a translator. I am Java Script, you are Java and the translator is cordova-admob-free plugin.
Edit: for example event(condition) to call appID, bannerID and etc inside C2 project please check on youtube video description, I put the download link for example capx.