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.
Non-text file editing is done in a 3d model or image editing app.
There are the rare engines with no editor to speak of -- where things are either done programmatically or other textual definitions -- but they're very very very few and far between.
The engine itself doesn't have a UI, but working with any major engine without using their editor is functionally impossible.