They don’t. ‘C’ get degrees is the mantra. They get passed on without knowledge. Sad. I watched a black girl get told to turn in a Calculus I exam with her name on it for a D- just to get passed on.
I never saw that although I understand it has become common. When I was in college there was no way the profs would do that. They would tell the student to take algebra again. I was able to do tutoring for the VA up through calculus, but all I ever did was algebra and trigonometry. I only did the tutoring for a few years because it was too depressing.