Traditional Medicare consists of Part A (hospitals), Part B (doctors) and Part D (drugs). Part A+B don't cover everything so you have a Medigap plan. I have Plan G which has very little paperwork. All up, I spend about $400/mo and I'm very happy with A+B+G+D.
With Medicare Advantage you sign over your Medicare rights+benefits to a private insurer. This may save you some money, especially early on. In fairness, not really a lot and the $0/mo plans are a scam. With Medicare Advantage, you will then have to argue with an insurance company for the rest of your life. You'll have to deal with preauthorizations and a restricted network.
With Traditional Medicare, what's covered is spelled out pretty clearly ahead of time. Docs know it. You know it. There's literally an app for that. With Medicare Advantage, medically necessary is at the discretion of the private insurance company.
Here is the scenario from a relative: he had a heart event which ended up needing a stent. He had to argue with Kaiser while this was going on. Kaiser is 240,000 people. He is one.
Medicare Advantage is very profitable.
It is possible to switch back from MA to TM which really revolves around your Medigap plan. You are guaranteed issue for Medigap plans for about 3 months before/after you turn 65. After that, you will have to undergo medical underwriting.