Pam, I didn't understand a lot of it until this past year and I still have a lot to learn. My brain hurts sometimes just thinking about it! Here is a link the Medicare.gov. Maybe it will help answer some of your questions:
https://www.medicare.gov/
Yes, on Medicare Advantage there is an in-network list that you are limited to go to and some areas don't even have an available MA network, so a Supplement plan is the only choice. With a Medicare Supplement Plan (aka Medigap), you can choose your own doctors and hospitals anywhere in the USA.
I would love to hear first-hand experiences from folks who either have a Supplement plan or Advantage plan. Are you happy with it? What don't you like about it? Thanks!