After adding your application to your Admob account, you need to create different ads for android and ios. When you say add an app in Admob, it manages you the question of android or iOS If you are going to advertise on both platforms, you have to add two applications for both Android and iOS.

     After adding your applications twice as android ios, you can complete the process by creating the desired type of advertisement (banner, transition, award winning, local advanced) in the ad units section. You should do this ad creation process separately for both android and ios.

     You can start showing ads in your application using the ad codes you create.