XAML is just a way of writing object graphs in XML. It is not tied to WPF or anything (heck, we use it to represent user data and style information within GraphML).

And yes, XAML is used by more than WPF at this point (Silverlight, UWP, and possibly more).

The markup is a pretty small part of XAML/WPF. The actual framework, controls, and rendering are the bulk of it.

Also Xamarin.Forms