Is there a descriptor for courses/classes you take without necessarily being in college?
In many circumstances, when the classes are organised by an institute of education such as a university, college or school, classes not held in the institution, or held for students who are not part of the institution’s registered student body, are extramural.
= organized by a college or university, etc. for people who are not students there:
Cambridge
From the Latin: extra + murus; “outside the walls”.