You don't need to use an IDE in order to write Xamarin.iOS or Android apps. You can just use a text editor (although Xamarin Studio is free). The MSBuild logic

You don't need to use an IDE in order to write Xamarin.iOS or Android apps. You can just use a text editor (although Xamarin Studio is free). The MSBuild logic, at least for Xamarin.Mac and Xamarin.iOS has been made completely open source, so all the logic to build your apps + the SDKs are all open source.