Note that Mono, the framework, is indeed dying. But Mono, the runtime, is now part of .Net and it's an alternative to CoreCLR, used for things like Xamarin/MAUI and Blazor.
Though most of the time, you don't need to know any of this, you just use .Net and it works on Windows, Linux, Android, Apple and in the browser.
Though most of the time, you don't need to know any of this, you just use .Net and it works on Windows, Linux, Android, Apple and in the browser.