My son played on a below 500 team in SoCal. Every starter moved onto D1 and pro or D2. It was a very competitive conference, many of the players were from out of the area on my son's team. It doesn't happen at every school, but the opportunity is there.
If you check out Chico State/SFSU etc. you can see that most of their players come from CA JC's. Some may imply that some schools are "feeder schools" and I would not buy into that thinking, the JC system is full of free agents and my son's east coast D1 team actually had more players from Cuesta (alleged feeder school) then CalPoly.
In the end it comes down to performing well and being seen. The CA JC leagues are scouted pretty good, especially in SoCal. They set the schedule to not conflict with D1 weekday games making it easy for coaches and scouts to watch games in addition to practices. They also schedule games before D1's can start practicing making it easy for the HC or RC to scout.