What are the options for Undeclared Physical Science..

My D has been admitted into undeclared physical science in UCSD warren college; which is the alternate choice. Her primary choice was mechanical engineering, but she did not get. She has also been admitted into SDSU and UCR in mechanical engineering. Since we live in SD, her preference is local school, particularly UCSD. Now we have a lot of questions, regarding this undeclared major.

How the undeclared major studies works? At what point, the students decide the interested majors? If the interested major is not possible or denied, what are all the options for the graduation in 4 years?

If she wants to pursue with mechanical engineering major in ucsd, is it possible with admitted in undeclared physical science?