The vast majority of non-text editing in game development isn't done in modeling or image manipulation apps, it's done via the game engine's editor. That's true whether you're using Unity, Unreal, Godot, or a homebrew engine.

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.