Is there somewhere where someone new to the ecosystem can get a simple introduction to all of these different terms and which ones are still relevant today? I looked into .NET somewhat recently and came away with the apparently mistaken impression that Mono was how .NET did cross-platform. I guess I must have been reading old docs, but I'm pretty sure they were at least semi-official.
Is there good documentation somewhere for getting set up to develop with modern .NET on Linux?
dot.net is the official starting point. It'll point you to getting setup with .NET dev on Linux. You can either use VSCode or Intellij Rider.
[1] https://avaloniaui.net/ [2] https://platform.uno/