Code is code, use a text editor.

3D assets can be made in your 3d program of choice.

Skeletons and animations are also somewhat standard. Use whatever tool you prefer.

Textures are bound to 3d assets, and use plenty of 3rd party tools.

USD makes scenes interchangeable. Use or write tools to fit your needs.

compute shaders are also somewhat standard. Work them out in python until you're happy with them.

Thats alot of it, if you really hate all game engine UIs. But the UI was good enough for the developer of the engine so mabie it's good enough as is.

I am confused by this as well (not a game developer). The engine is under the hood and has no UI. The UI is in the car cabin.

Non-text file editing is done in a 3d model or image editing app.