It varies. My son has lots of interests across a broad range of subjects. When my son was deciding between Harvard and Yale we spent a lot of time at Visitas and Buldog Days going to different departments to try and suss out how difficult it would be for a non-major/concentrator to get into upper level seminar classes in departments outside his major. Seemed like Harvard was more restrictive with departments having stricter rules and preferences for their majors and upper level students. For example, a senior concentrator would have priority, followed by a junior concentrator, etc.
Yale seemed much more flexible (with the exception of a very few programs like DS) with it coming down to: Talk to the prof, and as long as the professor OKs it…
That ended up being the deciding factor.