It’s very simple. You just use the latest mail direction file (a multi-gigabyte text file, btw), compare it against the portion of the most currently published Labeling List, and depending on the mail piece’s 11 digit routing string, mail class, and USPS induction facility, that will determine which individual carrier has whose mail on their route.
This comment is how I always hope my info dumps go when someone asks me a technical question about something I have good experience in using. 10/10 comment, love it.