My perspective is as someone who mainly used C# on the Mac with Unity. I was a professional C# developer at one point, but not on the Mac.

MonoDevelop, long ago, shipped with Unity on the Mac, and you'd use it for all your C# development. Xamarin Studio was the proprietary counterpart, and the selling point was that you could develop cross-platform mobile apps in C#. When Microsoft purchased Xamarin, Xamarin Studio was rebranded as Visual Studio, which is confusing.

So we are back to MonoDevelop if you want a free IDE for C# you can use on the Mac, with good C# support. VS Code has C# support through a plugin but it's not as good as MonoDevelop.