Riverside vs Orange, 2020

[Oryxslayer]

If you want to know why, the reason is based in who votes Dem in each county. The Inland Empire democratic party relies very heavily on Hispanics who drop off in off-year elections. While Orange Dems also depend on Hispanics, it is the Asian vote that powered the Blue wave down there in 2018. Asians didn't go as hard for Dems as they did for Clinton, but they mainly stayed blue (outside of local Asian GOP races) and mainly kept their turnout levels proportional to the rest of the population. So OC went blue whereas Riverside whites overpowered their Hispanic neighbors.

Despite the easy answer to 2018, your main question in regards to 2020 is still an interesting one. OC has more voters available to flip blue and has more  people moving in with democratic leanings. Riverside is less flexible. However, Riverside has been Blue for longer then OC, so it has a head start. If dems are able to get Hispanics to vote with higher rates for some reason, that will be felt more in Riverside than in OC.

[Oryxslayer]

Also CA 48th is a beach district, probably has that wealth and education to swing hard D in 2016.

Well they're pretty similar median income wise, but there is a big education difference.

CA48 is actually whiter than CA42, it's just that there's a big education gap, and Hispanics don't even come close to matching the white turnout of 42.