I would have imagined that in game dev you prototype first with a high level language and then you would write the math code. How much of game dev is actually math code, aside from critical components like rendering, physics, etc.?

No that is not how it is done, the math is written directly in C++.

If you have fast iterative build times, lots of assertions, and good debugger there is no reason to waste time writing it in another language first.

Almost every system uses basic vectors and matrices/quats, most gameplay code also uses them etc.

Consider the fact that QuakeC had a native vector datatype back in the day, despite the fact that all rendering and physics was done in native C code.

3D rotation is maths. How many 3D game objects don't rotate anything in gameplay?