For instance, med/business school classes?
There may be limitations for some cases. However, there are times when undergraduates can take graduate level courses.
I know an undergraduate who took at least 2 500 level courses last semester.
I don’t know about Medical or MBA classes, but I would assume that most graduate level courses in LSA are open to undergraduate students. I took a handful of graduate level Econ classes.
Did you take them after sufficient undergrad study? B/c taking grad classes requires some prereqs, perhaps many.
You can generally sign up for any class within your college unless they are restricted to a specific program. You can get overrides for others.