American health insurance is insurance in name only - picture health insurance models laid on top of your car:

Imagine your car gets totaled. Your insurer says, "Hey, we're going to pay out $25K for your vehicle. So you have a $1,000 deductible, so that's $24,000, and then your copay for a total loss is $2,000, so that brings us down to $22,000. For total losses, your coinsurance as your contribution for your vehicle coverage is 20%, which is $5,000, so here's a check for $17,000. But that's only if you're buying a Hyundai, otherwise the vehicle is out of network and you'll get a check for $8,500 instead."

> If you read the fine print of a health "insurance" plan at a large company, you might discover healthcare costs are directly covered by the employer and the insurance company just administers the plan according to "set rules".

Generally this is done by a TPA (third party administrator). In many ways you can do as you wish, but as insurers have already done the actuarial work, it's generally easier to use a plan and tweak it if desired (like "Give us this plan but pay for 1 massage/week") versus having to figure that out yourself.