The individual mandate part of the ACA was the part designed to reduce premiums. You need healthy participants in any health insurance scheme to subsidize unhealthy people.

That was eliminated by a Republican bill, the Tax Cuts and Jobs Act of 2017.

Yes, and that was years after the ACA took full effect and the rates did not decrease.

Most people get insurance through their employer, and most employer plans (at medium to large companies) are self funded by the company and merely administered by insurance companies.

That means the healthy participants had no effect on those plans whatsoever. Even at peak, the individual mandate had only cut the number of uninsured by half, and the effect on rates was negligible.

FWIW the state of california has its own individual mandate.