Crop factor doesn’t actually change the physical f-number of a lens, but it absolutely changes how that f-number behaves in practice, and that’s where people get tripped up a bit. The f-number itself is purely optical. It’s defined as the focal length divided by the diameter of the entrance pupil. So if you mount a 50mm f/2 lens on a full-frame camera and then mount that exact same lens on an APS-C body, it is still a 50mm f/2 lens.