Is it determined by the number of semesters completed? Do Freshman Athletes get to select before Seniors? Do Regent Scholars get to select before Seniors, but after Athletes?
I don’t know.
You say this like anything is going to be filled up before the seniors.
Nothing will be filled up before the seniors get there. And freshmen take completely different classes than seniors.
Don’t worry about it.
Just enroll in whatever berkeleytime tells you to enroll in first (Things that fill up phase 1) and you’ll get the classes you need. If you don’t get the exact classes you want, you have 4 years of classes that you need to take. That’s like 24 classes. At least 4 will be open first semester of freshman year.
Ignore prereqs. No one checks.
Honestly though, they probably just get randomly assigned in their super early time slot, or just all go at once.
Note that the UCB registration system allows departments to set priority levels for their classes. In some cases, seniors can only waitlist lower division courses in phase 1 or 2, so that frosh/soph/new-transfer students who actually need them for their majors can take them. I.e. departments can prevent non-major seniors from filling up the classes that their frosh/soph students need for their majors.
Example: https://eecs.berkeley.edu/resources/undergrads/cs/degree-reqs/enrollment-policy