It is different if you stay working more than 50% time after 65 but for Retirees; A&M employees that reach 65 years of age sign up for Medicare A & B with the feds. On the A&M benefits side you now sign up for the A&M 65+ plan which becomes supplemental to Medicare A & B for health and hospitilizaiton but is still your primary coverage for your prescription drug plan. (Since you don't have a medicare Part D plan.) You don't need to find a medicare Part D plan unless you want to because A&M's BCBS and related structure is your primary coverage on prescriptions.)

Medicare becomes the primary/first place your health provider bills for their services. Then, what medicare doesn't cover, the health provider bills to your secondary provider (BCBS) and they determine/pay what amount/% of the remaining balance they cover. After primary (medcare) and secondary (BCBS) have paid their parts, the final remaining balance is what you pay out of pocket.

When you pick up a prescription at your pharmacy, BCBS (or express scripts if you use that program) is the primary insurance and determines the copays, etc. just like it was before you went on medicare.

That is the very high level "co-ordination" of how things mesh together.