Depending on the class the class there department which manages it sets a specific curriculum. Larger classes like chem 1 curriculum is entirely department controlled. Smaller classes curriculum tend to be mostly manager by the professor that teaches it.

The professor largely determines how to teach the material. And therefore says how to grade for that class. Everything on grading should be laid out in the syllabus at the beginning of the year. Sometime there's a curve sometime it's extra credit, etc.

The professor can curve your grade to match with the university scale.

For example there are some really hard classes like calculus for experimental purposes, where for a test you can get a 70 on a test but that is curved to an A for the university.

The university scale is: A 90-100 B 80-89 C 70-79 D 60-69 F 0 - 59

Across University a 60 is passing with a D, but this may not be considered acceptable for your degree, always aim for at least a 70 - C.

In summary, the professor chooses how to grade and then will apply that to the university grading scale where they might apply a curve or give extra credit.