Some schools can be fairly transparent about how they admit along with # of admits by major and GPA but many are not.
SDSU lists the information of admitted students by major but not GPA admission information.
CSULB has admission and GPA information for enrolled students by major: https://www.csulb.edu/institutional-research-analytics/student-admission-data
Cal Poly SLO target projections shows the # of expected applicants and the # of spots available for each major but no information regarding GPA admission stats. https://content-calpoly-edu.s3.amazonaws.com/ir/1/images/2020-21%20Enrollment%20Projections.pdf
For the majority of schools, I have found little information and the information I have ascertained has required a lot of digging.
Private schools in general do not admit by major but there are always exceptions and the information can sometimes be found on their websites.
Since you referenced SDSU, all the Cal states admit by major and many offer Undeclared if undecided. SDSU admits into a “pre-major” for most of the majors listed and you are required to pass the pre-req courses before officially declaring the major.
Not all Cal states have “pre-majors” but it is more common with impacted campuses and majors.
For the UC’s, it will vary by campus and have listed the information below.
UCB:
Division (L&S, CNR, CoC, CED, CoE) matters for admission selectivity.
Within CoE (but not the other divisions), major matters for admission selectivity. Changing majors within the CoE after enrolling is not guaranteed, unless one is CoE undeclared.
Note that L&S admits students as undeclared; admission to capped majors (e.g. CS, economics, psychology, ORMS, statistics, art practice, and a few others) is by college GPA in prerequisite courses (and portfolio for art practice) after attending for a few semesters.
The business major is in a separate division and admits students in a competitive holistic process. Frosh intending business majors begin in another division (usually L&S), take the business major prerequisites, and apply (usually in their second years). They also need to take prerequisites for a backup major in case they are not admitted to the business major.
All students who apply to UC Berkeley and select a major within the College of Natural Resources are evaluated based on their application, not on the particular major they select.
Alternate majors not considered.
UCD:
Admission decisions are made based upon the qualifications of the applicant pool and the number of available spaces within each academic area:
• College of Agricultural and Environmental Sciences admits by college
• College of Biological Sciences admits by college
• College of Letters and Science admits by College but within specific divisions: ie. (Humanities, Arts and Cultural Studies; Mathematics and Physical Sciences; Social Sciences)
• College of Engineering admits by academic department
Students applying as “undeclared” or “undeclared/exploratory” are considered within the college/division to which they applied.
Applicants are encouraged to list an alternate major, but not in the
same area as the primary major (e.g., Computer Science Engineering with alternate Computer Science in the College of L&S). Occasionally, we admit to the alternate major.
UCI:
UCI admits into the University first and then into the major. In the case that UCI is unable to accommodate all qualified applicants in their first-choice major, those students who indicate a valid alternate major may be offered admission in that major or Undeclared.
UCLA:
For the College of Letters and Science, the applicant’s major is not considered during the review process.
The Henry Samueli School of Engineering and Applied Science admits students by declared major, with more emphasis on science and math programs.
The School of Nursing also places more emphasis on science and math programs and requires the submission of an additional supplemental application.
The School of the Arts and Architecture; Herb Alpert School of Music; and the School of Theater, Film and Television admit students by declared major (within the school), and put more emphasis on special talents through a review of portfolios and/or auditions, which are the most significant admission factors for these schools.
UCLA only guarantees review of an applicant’s first-choice major.
We typically do not admit to the alternate major
UCSD:
The campus does not admit students on the basis of academic major or choice of UC San Diego undergraduate college. Alternate majors are considered and capped majors are highly competitive. Also note: Capped majors require additional pre-req courses and specific GPA to be able to qualify if changing majors. Also if applying to a capped major, select an non-capped major as an alternate.
UCSB:
College of Letters and Sciences: Choice of major is not considered in selection to the College of Letters and Science. The exceptions to this rule are dance and music performance majors. Both majors require applicants to complete an audition in late January or early February.
College of Engineering: Students are selected by major for all engineering and computer science majors. Only applicants with a solid background in advanced high school mathematics will be considered for admission to engineering. This includes high grades in all math courses through grade 11 and enrollment in pre-calculus or higher in grade 12. A student not selected for their first choice major will be reviewed for admission to an alternate major outside of the College of Engineering if one was selected.
College of Creative Studies:
Applicants to the College of Creative Studies submit a supplementary application in addition to the general UC Application, which is reviewed by Creative Studies faculty. Students are selected within Creative Studies majors only. Applicants not selected for Creative Studies will automatically be considered for admission to the College of Letters and Science.
UCSC:
Important Note for Prospective Engineering Students: Choice of major does not influence the selection of first-year students, except for those applicants interested in a major offered by the Jack Baskin School of Engineering (BSOE). Freshmen who are interested in a BSOE program should be sure to indicate a BSOE proposed major. Students who do not indicate a BSOE program or who apply as undeclared might not be able to pursue a BSOE program.
UCR:
Admission by major but alternate/2nd choice major will be considered if applicant does not meet their first choice admission standards.
For Business: Freshmen students must apply to Pre-Business under the College of Humanities and Social Sciences (CHASS). The College breadth requirements and the prerequisites for a Business major are completed during the freshman and sophomore years. An application is submitted at the end of the sophomore year. Upon acceptance, students become Business majors and are then advised in the Business Department. Students from any academic major may also complete a Business minor.
UCI has some good admit data by major information here but the majority of the UC’s do not offer this kind of detail: https://www.oir.uci.edu/Data-Hub/Undergraduate-Admissions-Dashboard.php