I could be mistaken, but I can’t remember last time I heard of a C# project that wasn’t either targetting Windows as a platform (whether server or desktop) or running against some other Microsoft-thing like Xamarin.
In the early days of Mono, there were some attempts at using C# outside Microsoft circles, like building Gnome-apps with GTK#, but most of those projects have faded away.
I have around 500 c# .net lambdas. And several linux servers with .net on it. Also do almost all of my work On linux. Tho I’m starting to switch back to windows 11 with wsl2…