Yes, those who choose to go the Medicare Advantage (MA) Plan route still pay their Medicare Part B premiums to the feds. And CMS turns around and hands roughly $10k to the MA Plan for the MA Plan to handle the patient and billing, at least that is what I understand from the book I have. So it would seem to me that a Drs. office would deal with just one insurance company for those who are on MA.
Going the "Original Medicare" route like I (hopefully!) plan to do, uses the feds for Part A and Part B expenses, and then uses the Medigap Plan of my choosing for the residual (how much "residual" is covered depends on the Medigap letter plan chosen). So it would seem to me that the Drs. office has to deal with TWO paying organizations. So maybe that is one reason for some Drs. groups to avoid Original Medicare.
Some background on my PCP... They started out many years ago as a single-site business, with a handful of doctors. They later moved to a new location, and either created, or joined, a practice group, that grew to about 3 - 4 locations, all in the metro area. They also offered extended hours, for a price, at select sites.
Sometime recently without me realizing it, it all expanded to many sites, more than 10. The Dr. I used to go to (infrequently, I only go if there is a problem, and sometimes used a doc-in-a-box instead) quit practicing, I have seen another guy there before, he was OK. The site location is close by me, I know the facility, and I know the routine and what to expect there. As far as any Dr. there knowing me or history, all they have is notes on my last visits, which were far apart! No way would they remember me personally. So the "knowing me" medically aspect is not really important to me. It's as if it's more important to me, that I know them! I realize that may strike some as a bit odd, but I do not go for "routine" visits or tests or physicals or etc. But I'm probably more like the ~6.5 billion other people on this planet, than the "average" American.
I DID plan to check with their office a bit later this year to see if they said the magic words "accept assignment", or not. I was expecting that would be a "Yes we do". So much for that expectation!