M.A. people can still switch to traditional Medicare, i.e. parts A and B
but both those have limitations, for example only 80% of Part B expenses are covered. The uncovered 20% can get to be a very big number for some medical conditions.
One can purchase Medicare Supplement plans, also known as Medigap (NOT to be confused with Medicare Advantage), to fill in the uncovered costs and even reduce or eliminate the deductibles of traditonal Medicare. I have one such. But the good ones are expensive. All told, I'm paying $185 for part B + $320 Medicare Supplement + $40 Part D drug plan = $545/month.
Some people think Medicare is free or nominal cost. It is not.
Switching to a Medicare Supplement plan from a Medicare Advantage plan, or adding a Supplement plan when one has had only traditional Medicare can be subject to a medical exam and acceptance by the insurer, known as underwriting (Medicare Supplement plans are private insurance). One is free from this requirement only if they sign up for the Supplement plan within a 6 month window or something like that from when first getting on Medicare. There's also some one-year leeway IIRC for Medicare Advantage people trying to switch to a Supplement plan (i.e. you can try Advantage for one year or less), I forget the details of that.
Part A (Hospital Insurance) is free for most people (actually they paid in during their working lives through FICA payroll taxes). But those who don't have a full 40 quarters of lifetime covered earnings have to pay a big premium for Part A.