Medicare will typically pick up 80% of your Part A and Part B expenses. You have to come up with around 20% of costs. If you happen to be having Medicaid in addition to Medicare, Medicaid will pick most of these 20% expenses. This is your Original Medicare insurance coverage. You have to be having a separate Prescription Drugs insurance plan as to your Original Medicare insurance.
Private health insurance companies have contracts with Centers for Medicare and Medicaid Services [cms.gov] for Medicare Advantage plan. These contracts override the original Medicare insurance, and provide you with insurance plans, which may or may not include Prescriptions Drugs Plan, depending upon the plan you buy. There are three categories of plans:
Original Medicare: You hold ‘red and blue’ card having Part A and Part B stamped on in. You buy Prescription Drugs Plan from private insurance that has Part D contracts with the Centers of Medicare and Medicaid Services.
Medicare Advantage Plans: You sign up for this plan with a private insurance company having contract with Centers of Medicare and Medicaid Services. Note that there are rules of procedures and restrictions on enrollment to Medicare Advantage plans. If you are entering 65, start the process about three months earlier than you birth month.
Medicare Supplement Insurance. This is underlying insurance to Original Medicare insurance to pick up uncovered portions of the Original Medicare insurance.
For authoritative information, CMS published annual booklet Medicare & You. It is important to get familiar with it.
"We do not offer every plan available in your area. Any information we provide is limited to those plans we do offer in your area. Please contact Medicare.gov or 1-800-MEDICARE to get information on all of your options."