Many of the Medicare Advantage plans are HMO’s.
In California though, Kaiser is a huge HMO that is qualitatively different from, for example, a Humana HMO. Kaiser owns the hospitals, the labs, the physicians are on salary, it’s a completely self-contained medical network. Members use only Kaiser facilities, staffed by Kaiser employees. It’s been popular. The Cleveland Clinic uses a somewhat similar model that has gotten good reviews.
Other HMO’s, from my understanding, sign contracts with a variety of independent contractor providers and then limit their members to choosing only those providers (and usually require referrals).
The Kaiser type HMO is one of the alternative models that ACA supports. There has been a push for a move to either outcomes-based or flat fees based on diagnoses starting about 30 years ago when they began compensating hospitals set numbers of hospital days for an illness or procedure. So, if the hospital cuts you loose before three days had been used up, and appendicitis/appendectomy is allotted three days, the hospital could keep the difference as extra profits. If you got an infection and stayed 5 days, or required readmission, the hospital ate the losses. I’m not sure what happened to the program.
Now the insurance company is contacted on a regular basis, often daily, for approval for continued hospitalization, based on reported condition. If the insurance company says you know longer need to be in the hospital (i.e. they aren’t paying anymore), the doctor is notified to write an order for discharge. And people think their doctor decides when they get to leave the hospital? Ha, ha, ha.