Xamarin offers common APIs for platform-specific behaviors when using Xamarin.Forms, otherwise, when targeting Xamarin.iOS or Xamarin.Android, you're developing

Xamarin offers common APIs for platform-specific behaviors when using Xamarin.Forms, otherwise, when targeting Xamarin.iOS or Xamarin.Android, you're developing with pure-native APIs. You will need to use the native provider for whatever action you're executing. If you'd like to be able to reuse common concepts cross-platform, then you need to write your own interface and provider model. Xamarin is the equivalent of a thin wrapper/intercace over the platform-implementations. Xamarin doesn't really "do" much, it just calls the underlying API which maps to the specific behavior. The type and method signatures for Objective-C and Java are very near synonymous in Xamarin/C#.