I hear many misunderstandings of what Xamarin is and how it differs from native mobile.
There are two flavours of Xamarin: Xamarin Native and Xamarin Forms. They are very different.
The first offers a native mobile experience with the UI written in the native language for the device type. Therefore an understanding of the native tooling, languages, OSs and devices is required to do this type of mobile development. The advantage here is that Xamarin Native developers typically know native development but can also work across both iOS and Android in their respective native languages.
The second is an abstraction, does not have access to all native features and has more shared code between the device types than Xamarin Native. Forms is more suited to prototypes and line of business apps rather than consumer apps.
For an excellent overview of what Xamarin Native and Xamarin forms are and how they differ from native mobile, have a quick read of this: