How do you get back an impounded car in California?

That depends on why it was impounded.

At minimum, you'll have to go to the impound lot and pay the impound fee. There's probably a towing fee as well.

There may in some circumstances be a police hold on the vehicle, meaning you'll have to bring proof that you've cleared up whatever that is.

The first step would be to call the impound lot and ask them what you, personally, specifically need to do to get your specific car back, and then do whatever they tell you.