Looks like they are using IMGUI as dev tool: https://github.com/ocornut/imgui
It seems to me to be super high quality of a GUI library.
You can see it here being used in Rainbow Six: https://montreal.ubisoft.com/en/ubisoft-sponsors-user-interf...
The reason I like mobile and small desktop applications is partially for the tight cycles of build/test/fix/ship. Even writing it out now, the experience of working on any game of even moderate complexity must be entirely different, right?
If you work on a game, building ImGui tools isn’t a primary concern, it’s what you do to be able to easily test your stuff.