This is how a lot of big companies work. They self-insure. They set the rules, pay all the bills, and the insurance company just administers things.
If you work for a company that does this, be aware that the things you dislike about your insurance coverage may well have been designed that way by your employer, not the insurance company. Don't curse the insurance company when you don't like the benefits. Ask your manager and HR department why your employer chose that rule or limit.
Almost every sufficiently large employer does this. Instead of paying premiums to an insurance company which owns the risk (and also makes a profit from that) the large employer pays the actual costs of care, and a percentage to the insurance company for the administrative work (using their provider network, claims processing systems, etc.).
okay, but surely then they get reinsurance, no? also does it make sense for every huge company to have their own insurance department?
isn't this simply a service that big corps can buy from these huge insurers? so they end up as the payers, they can manage their own plan(s), but of course they actually don't even have one single actuary on payroll, right?
Yes they also typically buy reinsurance (insurance that covers individuals whose claims exceed some amount in a given year, like say $150,000). The company will have some people in HR who administer this program but they aren't duplicating a lot of what the insurance companies do -- that's why they pay for administrative services.
They rarely have actuaries on payroll but there are several boutique actuarial consultant who serve these companies (e.g. "given the geographical distribution of your employees and the higher use of services X, Y, Z, you would save $xxx if you choose insurer A over insurer B").
If you work for a company that does this, be aware that the things you dislike about your insurance coverage may well have been designed that way by your employer, not the insurance company. Don't curse the insurance company when you don't like the benefits. Ask your manager and HR department why your employer chose that rule or limit.