Medicare Start Date (updated)

I downloaded my Benefit Verification Letter from Social Security today. The letter said under Medicare Information that I am eligible for hospital insurance under Medicare beginning December 2018, and medical insurance under Medicare beginning January 2020. Hospital insurance is also Medicare Part A and there is no premium. Medical insurance is Medicare Part B. I’m assuming that since I do not receive a check from Social Security, I will get a bill for Part B premiums.

I still don’t understand who pays for what now that I have two insurance coverage. Does Medicare only pay for ESRD related expenses such as dialysis, or does it pay 80% for everything? I know that Medicare’s reimbursement rate is typically less than private insurance contracts so will my doctors take Medicare? Do I show providers both insurance cards and let them figure it out?

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I think I’m gonna have to read this entire PDF from CMS.