Games might be an interesting one to look at because they have a little bit of everything going on (physics, math, rendering, cross platform abstractions, window management, system calls, file management, memory management, etc...)
Take a look at these:
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