Xamarin.iOS is used for building applications for iOS with .NET, and contains the bindings to the iOS APIs mentioned previously. Xamarin.iOS uses ahead of time (AOT) compiling to compile the C# code to Advanced RISC Machines (ARM) assembly language. The Mono runtime runs along with the Objective-C runtime. Code that uses .NET namespaces, such as System.Linq or System.Net, will be executed by the Mono runtime, while code that uses iOS-specific namespaces will be executed by the Objective-C runtime. Both the Mono runtime and the Objective-C runtime will run on top of the Unix-like kernel, X is Not Unix (XNU) (https://en.wikipedia.org/wiki/XNU), which is developed by Apple. The following diagram shows an overview of the iOS architecture:
