SAT II exams are subject specific and depending upon the college, some require them, some recommend them and some do not consider them in their admission decisions. For the UC’s, they are recommended for some majors at some campuses. UCLA only has recommendations for Engineering applicants and not Biology major applicants however if you do well on the exam(s), they can enhance your application. If you are interested in taking some Subject tests, you should consider Biology E/M or Chemistry and Math 2 which are the common tests for STEM majors. I would take the subject tests after you have taken AP Biology, AP Chemistry or Algebra 2 so the information is fresh. You can go to your local library and find some SAT subject test review books or practice tests are available on the CB website.
You take them separately from the SAT I and/or ACT and you can take up to 3 Subject tests in one sitting.
More information can be found on the College Board website: https://collegereadiness.collegeboard.org/sat-subject-tests/subjects