MS should push Visual Studio Code, as that's properly multi-platform, and forget Xamarin. Now that the .NET CoreCLR is open source.
Monodevelop is an awful piece of software.
Monodevelop is an awful piece of software.
(Actually to be fair, you'll find the Atom family of tools and related plugins to be not terrible for writing C# code. We can only hope that VSC continues to grow and spread and monodevelop fades away into the 'what were we even thinking when we made this?' it deserves to)
For Mac and Windows, you can download Xamarin Studio which is a bundle of MonoDevelop along with Xamarin iOS/Android plugins and branding. Note that a Xamarin license is only required if you develop iOS/Android projects, all other project types are free without restrictions.
Unity3d has been using Xamarin's .Net to build for Android and iOS (and many other platforms) for years now.
On iOS the situation is even worse you will need to have an AOT compiler for C# in order to be able to ship iOS apps in the Apple Store.
You are making a confusion here. Unity doesn't use Xamarin at all, they have an old license (like really old, since before Xamarin even existed) for Mono for iOS and Android.