Waitlisted at UCSD & UCD

Hi,

I am a high school senior. I recently got waitlisted at UC Davis and UC San Diego. At Davis, I got waitlisted for my second major which is material science engineering. At UCSD, I applied for first major Electrical Engineering and second major ECE. The UCSD portal is asking me for an alternate second major. There are limited options to choose from for alternate second major. The only engineering option I saw was nano engineering. I also saw applied math, physics + smth, and straight undeclared. Wdyt I should I choose? Does anyone know if it’s hard to transfer from a non engineering major to electrical engineering or ece at UCSD? Do I have any chances to get off these waitlists?

I was also wondering what are the UC waitlists based off of? Are decisions based on available spots within specific majors, or do they consider the number of students from particular high schools?

Thank you