Yes, it is very important to understand the difference in Medicare traditional (Part A, Part B, Part D) and Medicare Advantage plans (Part C) where a private insurance company takes over and approval is required for visits and tests and you must stay in their network.
One does well to check the Medicare site each year and compare plans. Because insurance companies make small changes that impact final price!
And the plans available depend on where you live.
Many years ago, I fortunately went with traditional Medicare and bought a Medigap policy to pay the 80% not covered by Medicare. Then less than 2 years ago I was diagnosed with multifocal lung cancer, and so far have had two surgeries and one radiation treatment and more are planned as this cancer is like "chronic". I also had a cardiac blockage and needed a stent. Traditional Medicare allows me to self-refer so I said goodby to local doctors (and they lacked experience with my type of cancer and could not have offered latest in surgery options) and I went to Mayo Clinic (a 2-hour drive). I have never had to get insurance company permission for a test or to see any kind of specialist. In 2011 I needed ear surgery. I was living in Florida at the itme so I just went to the best place. I have never seen a bill for all the expensive treatments, not for any visit of any kind. If I think I need to see a specialist I just make an appointment. I pay $300/month premium for my Medigap policy but I can go anywhere in the US that accepts Medicare so good for vacation. I would never change plans. I have a simple Part D plan $17/mo premium. I have not needed any cancer drugs so I can't address that. But I can aways change the Part D plan if I should need any. From what I know about Medicare Advantage plans, I think they should be eliminated, and not offered.