Holy crap, all these answers and hardly anything about how health insurance is supposed to work.

Basically, most people have health insurance. With the Affordable Care Act (aka Obamacare) from years ago, it's basically required but getting care is simpler (you can't be denied a plan for a preexisting condition, for example. But it's hella expensive. It's also typically tied to your employer as part of your compensation package like retirement contributions, which means if you change employers there's a good chance you need to change insurance and even doctors.

ANYWAY, say you have procedures done. Insurance companies typically have contracted amounts for stuff with each provider (a "discount" from insane prices nobody actually pays). You typically pay the first however many dollars, depending on what you're having done and how your plan works. Eventually, you'll reach a dollar amount that's your maximum for the year, and from there insurance generally covers everything they normally would

Some people also have Medicare (ages 65+) and/or Medicaid (based on income or disability). Some people have private insurance on top of it. My daughter was born extremely early and stayed in the hospital for months. Her very low birth weight was a qualifying disability for Medicaid, and she was on my work health insurance. Claims would go through work insurance first, and any remaining costs (deductible) would be passed to Medicaid. If there was anything left, I'd be responsible for that. But I don't think that ever happened

Obviously, coverage is different for different people - different employers have different plans that cover different things differently. But in theory, that's how it should work.