When does on campus recruiting/do I apply for internships IB

I plan on transferring to USC as a junior. I am currently a freshmen at ucsd. Am I going to be too late in transferring? My understanding is that in order to get a post grad job you need to be a summer intern in the summer of a rising senior. And in order to get that internship you apply during your junior year and interviews occur Jan-Feb. if that is the case, why are people telling me it’ll be too late and a waste if time?