What insurance companies come under the ACA?

I need to understand this a little better. Someone on fb posted that only 5 of the 30 cancer hospitals in the US will accept the ACA. But the ACA isn’t an insurer. It just set parameters on what existing insurance companies can and can’t do, right?
Also, they’re trying to up enrollment on all the insurance carriers to reduce premiums, right?
So it affects ALL insurance providers, include BCBS Silver plan? (On this post there was a claim that the Mayo Clinic only accepts BCBS Silver plans, which is not true, of course.)