20 years ago, my shop was next door to the local UPS depot.
The process then was everything went to one of 6 hubs around the country. The closest one to IL was in KY at that time. If a package dropped off in Freeport was being shipped to Freeport, and someone in shipping caught that, the package was put on the proper truck for delivery.

But if no one caught it, it was sent to the UPS airport in Rockford and loaded up on a plane. The next morning it was at the nearest hub where it went through sorting and was flew out by plane to the nearest hub to its delivery point, or if that was the nearest hub to its delivery point, it was sorted to be sent to the nearest UPS airport (to the deliver point) then sent to the depot to its delivery point. There it was placed on a truck to be delivered.

That Freeport package was driven to the Rockford airport, flew to KY, sorted there and sent back to the Rockford airport sorted and sent to the Freeport depot, placed on the truck for delivery. I doubt the process has changed much, other then I heard that everything now has to go through the full process. The depot people are not suppose to pull something off of one truck and transfer it to a different truck at the depot, it now has to go to the airport, go to the hub, return back to the original airport, then return back to the original depot and then put on the correct truck.

I've been in that Freeport depot during the Christmas delivery season. Its a wonder how anything gets where its going! The store closed at 5PM. Everything that came in on the 20 trucks had to be loaded onto the semi trailers, and the semi had to leave for Rockford by 7 pm. That loaded semi returned from the Rockford airport just before 11 pm (if the roads were good), then the semi had to be unloaded, and everything had to be sorted and put on the correct truck for delivery. Those 20 trucks left the shop at 8 am, and most didn't return until very close to 5 pm, some were after 5.