We belong to the non-profit Kaiser Permanente HMO (only available in some states, FYI), so our Medicare enrollment seamlessly rolled into a Medicare Advantage plan. Kaiser has a comprehensive, closed system with its own doctors, specialists, hospitals and pharmacies, so as long as you are within reasonable distance from one, it's a good choice that has worked out well for us.
On rare occasions, if there is a someone with a really serious or complex injury/illness, Kaiser will have them transferred to a private specialist or surgeon, but I don't know of anyone who has had that happen. If you have an accident and are unable to give your info to the EMTs, and are taken to a local ER, Kaiser will arrange payment once notified, but all follow-up is, I believe, given at Kaiser after ER discharge.