From my experience... it varies widely. The university will have a standard, the department will, the professor will.

I had experience taking undergraduate courses at 4 institutions in the USA and of course many departments.

I had some professors that expected the grades to fall like grade school did with 90-100 % being a high percentage. Others gave 4-6 difficult multi part problems and graded on a transparent curve. Others gave difficult problems and were not transparent about the grading scheme.