I can't speak Frictional's code quality, but historically released titles tend to be a tangle of hacks and short-term decisions that hammer things into working or being performant, coming at the expense of future developers and correctness.

I do agree that the field is full of great code to reference, though.

I'm a huge fan of Godot's codebase. It's well written, and easily understood both architecturally, and in-method. https://github.com/godotengine/godot