I tend to call this 'the Unity solution' or 'the editor solution.' It is not a runnable or deployable app, but you can attach the Visual Studio debugger to the Unity editor by pressing Start in Visual Studio, and the the play button in Unity. You use this for typing code and trying things out.
Typically, when you build an app for Mixed Reality, you have a solution with C# code that you use while working inside the Unity Editor. I hope this fills the gap and shows the whole road to IL2CPP debugging in an easy to find article.
They all have part of the story but not everything. You can find how you can do it in various articles, forum posts, etc. This does not mean you can't develop in C# anymore - you still do, but debugging becomes quite a bit more complicated. Some time ago, Unity announced they would do away with the. I also have cursed them to the seventh ring of hell for the way the move (too) fast and break things. It allows me to weave magic and create awesome HoloLens and Windows Mixed Reality apps with an ease that defies imagination for someone who never tried it.
