Correct. Plan B premium tied to AGI on tax return

As you said, the Plan C premium = to Plan B premium, in most plans / large retirees counties, directly deducted from your SS benefit.  Some counties Plan C still requires Additional premium payment - that part is paid to the insurance company if I understand it correctly.  In other words, the member would pay a premium in the form of deduction from Social Security benefit, PLUS an additional premium to insurance company.  We have a friend in SFO is on Kaiser Plan C, he still has to pay some $$ on top of the deduction from his Social Security benefit.  In Florida Broward and Miami-Dade counties, virtually No Plan C has additional premium. 

Each year's open enrollment time, Medicare will send you a thick book listing all plans in your county for you to do comparison shopping, as you can only change plan during the Open Enrollment time frame.