How are classes enrolled to undergrad students at Penn?

Hey guys,
So I am considering applying for computer science/engineering in SEAS, but I am also very interested in other fields within SEAS, but also outside. Primarily physics in CAS.

How exactly are the classes allotted to us and how flexible is it?

Unfortunately, there aren’t any minors interesting to me, so all my studies in subjects of interest outside of my major would have to come through allocated classes.

Thanks!