Even though I'm not quite Medicare age yet, the past couple of years I was on a quest to understand it all.
'Medicare Advantage', or 'Part C' in my opinion shouldnt even be called Medicare, but its meant to be confusing. Basically 'Advantage' replaces Medicare. CMS (the Medicare Fund) basically pays the insurance company over $1,000/mo for each Advantage participant, and basically say "Okay, you are in charge of their care." So the insurance company is in charge of which doctor/facility network you can see, what they will cover, and when, and to them, it's all about profit, so they can delay, deny, defer. Also the plan can change during the year. Often the network is limited to your County...
Yes, you are still required to pay the Part B monthly premium.
There are independent insurance brokers that charge you nothing to work with them, they are paid commission by the insurance carrier on the back end, but you never see it, and to work with a good one that understands both Medi-Gap (a.k.a Supplement Plans) they can fully explain the details, pros and cons of each.
I did breeze through the at&t Advantage Plan w/UHC a couple of weeks back, sure, they all sound appealing, but they can change often, heck, at&t can choose to drop it or change (like they've done with benefits in general).
Medicare Supplement Plans are also run by insurance companies, however, the Supp Plans are under the strict rules of Medicare. So what your doctor says is medically necessary, Medicare Part A (hospital in patient) and Part B (Doctors, Services, Tests, etc) will pay the 80%, and then the Supplement Plan MUST pay the remainder, per the terms of the Plan (G, N, etc). The insurance company underneath the Supp plan has no say, they have to pay. And, all the Supp are the same so whther it's Blue Cross, UHC, Kaiser, etc., a Plan G is a Plan G is a Plan G, so a cheaper premium isn't anything different from company A to company B...