As far as I can tell, there are two types of health insurance here in California: PPO and HMO.
HMO is most like the NHS - you have a GP, they are your gatekeeper for health care. If your GP approves something, they'll sort it out. If they don't approve, well that's too bad.
The alternative is the PPO. Here you don't have a GP, and you have to pick what doctor/whatever to go to whenever you need something. Roughly, you'll pay 10% of the cost (which will be expensive!) if it's "in network" ... that is, somebody approved by your PPO provider ... and 30% of the cost if it's "out of network".
There are other things that help make this work: you can set up a "Health Savings Account" and pay money into it before tax. Then you can use that to pay for the more routine parts of your health insurance.
Oh, of course, there are other possibilities. You could pay for everything out of pocket.
Generally it seems quite odd to me that everything to do with healthcare is called "insurance" here, even for stuff that is fairly routine. It's like your house insurance covering having to paint your house or clean your windows, or your car insurance covering new tires (sic) and even petrol.
Everything is in stark contrast to the NHS, which Just Works. (Except when it doesn't, or when it works slowly.)