I’ve mistakenly believed that all UCs admit by major for transfer students, but I’ve come to learn this isn’t the case.
Do any admit by major? Or is it by school for all of them? How does this impact admissions?
Usually its both. I know at UCSD and UCLA. You have to get by the UC minimum requirements first. Once you pass that gate, your application is sent to your department for the final gate. Pass through and your in, don’t and you are rejected unless you have an alternative major. Same process, accepted or rejected. So the major has to accept you. You wont usually be a floating major unless you have a spectacular reason.
https://www.universityofcalifornia.edu/infocenter/transfers-major indicates that admission selectivity does vary by major.
However, in some cases, you may still be admitted as undeclared in the division (e.g. UCB College of Letters and Science) and have to complete remaining prerequisites and declare major after transfer.