From my understanding, .NET, C# F# are all open source. The main thing in the tool chain that isnt is Visual Studio but most people can get by with VS Code
Highly recommend using JetBrains Rider instead if you want the best IDE experience. It's not a Microsoft product & is used by a large percentage of .NET devs.
You are correct to mention that especially since theyve eased up on their licensing recently, I had in mind that it was still paid for software like Visual Studio. Rider is much better for .NET than VS code!
There's also mono
yes a key part of the ecosystem! Roslyn is open source as well IIRC