So everyone gets Medicare A (hospitalization) who qualifies for Medicare (~40 quarters of SSI taxes).

You get Medicare B (hospital or physician administered drugs like chemotherapy) if you pay the premium.

Everyone should get Medigap, as Medicare A + B only pay 80% (or 90%?) of costs, so Medigap covers most of your obligation under A+B. Those with very low incomes can get a LLS (low income supplement where the government covers the gap).

Medicare C is Medicare Advantage is HMO Advantage. It's basically the government paying premiums to an HMO Medicare plan instead of providing the benefit directly themselves. It covers Medicare A+B+Medigap.

Medicare D is prescription drugs (drug you take home and give yourself). Your premiums are paid by the government but offered by private insurers. The choice here (and there are calculators) are more complex as there are multiples plans to choose from and they are updated each year and it depends on what drugs you're on or think you'll be on.