If you have the Swiss Matura you can enroll whenever you want, whereever you want and major in whatever you want. There is not really an admission process.
I'm pretty sure it's the same for international students, it's definitely the same for EU citizens -- if you meet the requirements you're in.
There might be a quota for overseas students, that they can't accept everyone -- if this is they case the admission decision would ultimately come down to APs/IB, etc.
I don't believe this is the case -- if you meet the requirements you can expect to be accepted!
They just separate the wheat from the chaff during the first two semesters: at Uni Zurich in some majors 50%-60% of the students drop out.
