When we became responsible for DH's father's care -- literally overnight, when he was involved in a bad car accident -- we were absolutely clueless about Medicare and Tricare (he was retired military).
The medical bills came pouring in, some marked "final notice" or other dire warning. His Medicare and Tricare statements were pages* long (FIL had chronic health conditions to begin with, but once all his hospitalizations and treatments for the car crash kicked in ... oh my).
(*Ed. to add: one of his monthly statements -- from either Medicare or Tricare, I forget -- was Twenty.Four.Pages.Long.)
In a panic, I paid one of the most persistent bills, for ambulance and EMT services after the crash.
Sure enough, a few months(!) later, pouring over FIL's huuuge Tricare statement, it showed Tricare paid that same ambulance bill.
I think we managed to have our payment refunded, but that time was such a blur ...
But I did learn not to panic when the bills came in, it took several months before Medicare and the supplemental policy caught up. In the meantime, some providers billed FIL directly. I just put those bills in a big ol' pile and checked them off when they eventually showed up as paid on the Medicare or the Tricare statement. And IIRC, they were all paid ... eventually.