Xamarin extends the .NET platform with tools and libraries specifically for building apps on iOS, Android, macOS, and more.
These are some things that Xamarin adds to the .NET platform:

1. Base framework for accessing native features
2. Extensible Markup Language, known as XAML, for building dynamic mobile apps using C#
3. Libraries for common patterns, such as Model View ViewModel(MVVM)
4. Platform specific libraries that includes access to APIs from Google, Apple, Facebook, and more to add rich capabilities
5. Editor extensions to provide syntax highlighting, code completion, designers, and other functionality specifically for developing mobile pages

Xamarin is an open-source platform for building modern and performant applications for iOS, Android, and Windows with . NET. Xamarin is an abstraction layer that manages communication of shared code with underlying platform code.