If you just want to get into .NET (C# or F#) on non-Windows platforms, the latest .NET release (at the time of writing, 8.0) is what you want. The development experience is good these days.

For a historic summary of why it used to be a confusing situation (up until maybe 2020), see https://two-wrongs.com/dotnet-on-non-windows-platforms-brief...

That also has some background on why the Mono project found itself in a weird spot.