If you are referring to FedEx Ground, all of the trucks pulling their trailers are smaller contractor fleets who hire drivers and teams to run them. Each contractor pays their drivers differently and offers different benefits.
I'll follow up with started info on FedEx Freight: doubles/triples endorsement required, hazmat required. This is typically a nighttime position, although there are regions that also have daytime drivers. The PPM can vary, but in my area, I know that they pay .64/mile, with an additional .02(?) for triples.