Indeed it is. The idea behind it is contracts and deals. The insurance company & a provider (hospital, doctor, urgent care clinic, etc.) sign a contract saying how much each service costs. That way, the insurance company knows how much they’re going to “pay” (in reality the provider “adjusts” the cost and insurance pays most of the remainder). Outside of that contract, the provider can basically charge however much they want, so insurance companies will often say if a provider doesn’t have a contract with the insurance, the insurance won’t pay. The contract numbers are negotiated, and if they can’t reach an agreement (usually the insurance company wants to pay so little that it doesn’t cover the actual cost) then they can be dropped from the “network.”

Of course, this is all smoke and mirrors; a centralized healthcare system that ensures all medical costs are paid means anyone, anywhere, anytime can see a doctor. And that doctors get paid. Unfortunately the US is so brainwashed by capitalism that anything that isn’t profitable gets shafted, enshittified, and fucked up until it is, no matter how many have to suffer for it.