You might not be able to ride on freeways and expressways in California, but there are way more roads than those to get around on, not to mention dedicated bike paths and greenways, all of which will be much, much nicer places to ride than alongside 8+ lanes of high speed traffic.
There are countless resources available to you, just do a web search. Here is one of the top hits:
Map & Routes - CalBike - https://www.calbike.org/go_for_a_ride/map_routes/ But even if you don't want to do any prep at all, just open Google Maps on your phone, pick where you want to go and click the bicycle button. It will automatically plot a route for you that avoids freeways and (sometimes, not always) will lead you down dedicated bike paths and rail trails. California isn't some remote, poorly-mapped, provincial backwater, it's one of the wealthiest and most populous states in the wealthiest country in the world, home of the Google headquarters itself, filled with millions of bicycle enthusiasts etc. You should have no problems finding a safe and legal route to take you wherever you want to go.
Now occasionally, in extremely bicycle-hostile parts of America, you might find yourself in a situation where you need to ride illegally. This usually happens where there is a river crossing and there is only one bridge and that bridge is freeway only. A couple of times i've just said "fuck it" and take the whole damn lane and the hell with any of the polluting-ass fossil-fuel burning scumbags who are going to be inconvenienced. Other times you might find someone with a truck who is happy to throw your bike on the back and take you across the bridge or through some weird-ass intersection (try asking at a gas station), or maybe you can wait for a public bus and hang your bike on the front, or whatever. Sometimes you just gotta do a 20km detour to find a bridge that you are able to cycle across, or give up on crossing that river altogether. If you're on the road for the long-term, doesn't really matter, you don't have anywhere to be, right?
