The whole point of .NET-Core was to remove all the (largely desktop-oriented) platform-specific dependencies that tied it to Windows, so you could run server-oriented .net programs on Linux. So no, afaik you can't simply run GUI apps built with .Net on Linux desktops - that's the reason Mono wasn't simply killed, because it covers that niche (which wouldn't even exist, were it not for Mono/Xamarin's efforts back then. But I digress...). Nowadays there are a few other attempts at providing that UI layer.
.net Core still has Windows Forms thoguh? At least I (for kicks) migrated one of my old .net 4.something projects to .net core and it still works and shows the classic Windows Forms GUI.
.Net Core on Windows has support for loading assemblies that reference COM interfaces and the win32 API, along with other things that aren’t supported elsewhere like C++/CLI.
That’s why loading System.Windows.Forms still works, it’s not part of .Net 5+, but it can still load the assemblies on Windows (they still use GDI, etc under the hood).
