Is this Medicaid or Medicare?
If you go to the Medicare website, they have a list of all the Advantage plans in your area and what they cover. There's big range. They are not all $0. They do not all cover the same things. They do not all have a wide network.
Essentially, you still pay for Part B, and the Advantage plan expands on that. Take a look at what is available to you. You can see what your prescriptions will cost through each plan, too. The ones that are PPOs instead of HMOs seem to have pretty high co-pays for out of network care.
My dh is retiring this summer, so we'll transition onto Medicare. He has supplemental insurance through his retirement plan, but I'm going to use an Advantage plan, and there are a lot of options in the area we are moving to.