Xamarin have two commercial products, Xamarin.iOS for iOS development and Xamarin.Android. The Xamarin frameworks allow you to write applications using C# and the .NET framework. For each platform Xamarin provide bindings to the native platform APIs. As a result Xamarin applications make use of the native UI for each mobile platform. Xamarin do not provide a Windows Phone product because the C# and .NET code used for Android and iOS development is directly portable to Windows Phone.
The PropertyCross implementation makes use of the Model View Presenter (MVP) pattern in order to share as much UI logic as possible. The Model and Presenter code is shared across all three mobile platforms, with the View code, which makes use of native UI components, being distinct for each platform. This is reflected in the code-sharing statistics.
To view the code and detailed build steps, see the github source.