The architecture of Xamarin.Forms 

Xamarin.Forms is more or less just an abstract layer on top of each platform. Xamarin.Forms has a shared layer, which is used by all platforms, as well as a platform-specific layer. The platform-specific layer contains renderers. A renderer is a class that maps a Xamarin.Forms control into a platform-specific native control. Each Xamarin.Forms control has a platform-specific renderer.

The following diagram illustrates how an entry control in Xamarin.Forms is rendered to a UITextField control from the UIKit namespace when the shared Xamarin.Forms code is used in an iOS app. The same code in Android renders an EditText control from the Android.Widget namespace: