@Fish125, are you sure you aren’t thinking about UCSD? UCSB suggests that you provide as much MATH info as you can if you are applying to the College of ENGINEERING. They would like to see math subject tests, math AP tests, high math scores on ACT/SAT etc. Similarly, if you are applying to the College of Creative Studies, they would like to see a subject test in your area of interest.
At UCSB, Biology is in the College of Letters and Sciences and they accept you into that college regardless of your major or whether you are declared or not. They do not ask for subject tests for the College of Letters & Sciences.
At UCSD, Biological Sciences is a capped major so that might be where you read about submitting extra information (ie. Subject tests) to help show competency. https://admissions.ucsd.edu/freshmen/requirements.html
@Needyourhelp we just returned from UCD this this weekend. We spent three days talking to academic advisors in different departments. We were told multiple times that they don’t have impacted majors and that you can change majors and move between colleges once you get there. Most of the academic advisors expected kids to change majors. If you are, let’s say, a Psych major and want to change into Bio Sci, you will have to do more paperwork because it is in a different college. You will need to have taken some Bio Sci classes and shown competency before changing majors. We were told that the College of Bio Sci does not allow students to stay in their college if they get a D in one of their classes. If that happens, the students are asked to change majors - usually to one in the College of Letters and Sciences.
Regarding declaring or not declaring a major, at UCSB (and probably other UCs as well) you might get accepted as undeclared but priority in class registration goes to those that are declared. For example, if you went in as undeclared you will have more difficulty getting your lower division Bio Sci classes compared to someone who is declared as a Bio Sci major. That isn’t to say you won’t get in a class but you will need to be more flexible about what time you take the class.
At UCD, registration for classes is based upon how many units you have so seniors register first, then juniors, etc. https://registrar.ucdavis.edu/registration/register/pass-times.cfm I could be wrong but my understanding of the Pass system is that Pass 1 is for classes in your major and Pass 2 is for classes outside of your major.