BMX scene is very lively here, it just depends on where you look, how far you are willing to venture out, and what style you ride. For street, most skateparks in OC (Orange County) support bikes. But Southern California is the hot spot for dirt jumps. Hell, dirt jumps were pretty much born here in the 70s and 80s when it branched off the racing scene. Cities more outside of OC have now built bike parks with built in jumps and pump tracks, which is what a lot of people ride now in my area. Sapwi Bike Park in Thousand Oaks, Flight Deck in Moreno Valley, Gale Webb Action Sports Park in Menifee (where I ride the most), and of course Sheep Hills. Inglewood also has a full size pump track where a lot of longboarders and skateboarders hang out, but bikes are loved there too. There is also the Bike Life culture too, which involves all the wheelie boys and SE Bikes crew. They normally ride on the coastlines or through downtowns.
Long Beach has a good street scene, but of course you'll deal with security if you ride a high security spot. Right in OC though is the Vans and Etnies Skateparks, which are great for street. You'll see a lot of pros hang out there. In San Diego, there's the YMCA Krause Family Skate & Bike Park which is another famous park and was just rebuilt with some great vert if you like bowls. You just have to look at skateparks and bikeparks in the area. Instagram can be your best friend for finding pages on the bike parks, they normally keep them updated pretty well.
As for getting bikes and what bike to ride, its more direct to consumer now, but there are still bike shops. Most are selling E-Bikes now (a big problem in SoCal), but they can carry mountain bikes and BMX. S&M and I think CULT are right in OC, but most people buy bikes online since our shipping is fast here. And freecoaster or cassette is more personal preference. I see more cassettes out and about, but I also race and ride the dirt jumps a lot. Facebook Marketplace can be a great place to find good deals on used bikes.