Short answer: When you retire, your active employer coverage usually ends, which triggers an 8-month Special Enrollment Period to sign up for Medicare Part B without penalty. Some employers offer retiree coverage or an HRA that works alongside Medicare.
Retirement ends your active-employee group coverage, and that loss opens a Special Enrollment Period to enroll in Medicare Part B (and Part D) without a late-enrollment penalty; you generally have up to 8 months, though enrolling before coverage ends avoids a gap. Once on Medicare, you choose Original Medicare (often with Part D and a Medigap policy) or a Medicare Advantage plan. Some employers offer retiree health coverage or fund a Medicare Premium Reimbursement Arrangement/retiree HRA that helps pay Medicare premiums and supplements. COBRA is also available, but it does not delay the Part B enrollment deadline, so don’t rely on COBRA alone past 65.