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ADMob is supported in Android and iOS, there are Xamarin.Firebase.iOS.AdMob and Xamarin.Firebase.Ads packages from Nuget. You could create a view to display AdMob in Xamarin.Forms, and use customrenderer on each platform.
First,you have to setup an Admob account and create one app for Android and one for iOS.
Second, you need to create the Ad Unit of these two apps.
Finally, you need to implement the view on each platform.
Here is a link about Android, you could have a try to search for "AdMob In Xamarin.Forms" with search engine, there are some blogs about this.
Best Regards,
Wenyan Zhang
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