Cozie You mean how does health insurance work? Basically, we pay them a premium and they pay a percentage of our medical bills. My insurance (not ACA, I'm covered as a retired Govt employee) pays 80% of certain services, other services I pay a co-pay and they pay the rest. Certain services are paid for 100% by insurance. It's a bit more complicated than that, of course. But for example my mammogram, smears, & yearly physical are all covered 100% as are all immunizations. I pay $25.00 when I see my GP for any reason and I pay $35.00 to see a specialist, insurance pays the rest for the office visit. They also pay 80% of any lab fees or procedures. Hospital bills are a bit different, but as a rule I pay 20% and insurance pays the rest.
Insurance companies are involved because they make huge profits. But those profits are based on a bit of a gamble. People like me, who are rarely ill, make money for them because I generally pay more in premiums than I receive back in benefits. DH, on the other hand, has cost his insurance company probably over a million dollars in benefits paid out. They'll never get that much in premiums from him.
Insurance companies want to make the biggest possible profit, therefore they have a vested interest in the ACA and want it to be as 'restrictive' as possible (i.e. pre-existing conditions, benefit caps, selective coverage, etc) whilst getting as much money in ACA subsidies and tax incentives as they possibly can.
I really wish we could have an NHS style program. But the word 'socialism' is second only to 'communism' in 'dirty words' when it comes to politics.