The documentation Integrating ads using Google AdMob [GDevelop wiki] is a bit confusing on this topic. So can I? Or regardless of Test mode or not, I have to build an APK to test the Banners?
Test mode is for not getting banned from admob. When trying out admob in test mode add show up but you don’t get revenue. You have to use that when testing a mobile build because else admob detects you are watching ads to give yourself revenue which is not allowed.
Yes you need to build an apk because admob is a mobile only library.