Godot is written in C++ It may have some GDScript in there, but I don't think so. The sourcecode is available: https://github.com/godotengine/godot
The C++ code there (at least in the editor directory) initializes and configures godot ui components that the editor is made of
The C++ code there (at least in the editor directory) initializes and configures godot ui components that the editor is made of