unicornlox The Mirror is a compiled game as you put it. It ships with export templates. It doesn't use any of the Godot editor code. All of the editor mechanics like the gizmos, hierarchy, and inspector are remade from scratch.
Yes, we made our own visual scripting (or, specifically, I did). Yes, it's Godot 4. You can find docs on how to use our visual scripting here https://docs.themirror.space/docs/category/visual-scripting feel free to ask any questions.