I think you misunderstand: in California, all candidates, from all parties, run in a single primary (there is no separate "Democratic primary" or "Republican primary"). The top two candidates for each office from that primary are the only ones who advance to the general election. Since California is so heavily Democratic, often what happens is the general election is Democrat vs. Democrat. That was the case in 2018, when Kevin de Leon (another Democrat) ran against Feinstein and lost. Though not by a landslide, only by about 9 points.