A USPS distribution center is a sorting facility and a large building where packages and mail goes to after you drop it off at your local post office.
When you mail something it leaves the local post office or area and goes on a truck to a large distribution center where the mail or packages are then sorted and then put on trucks or planes that are going to the destination.
Your mail and package may go through a few more distribution centers before it actually reaches the destination.
If you live in the same state or within a hundred miles or so then you may get the package or mail within the next day.
But if you live in a different state or 1000 miles or more away it may take 4 to 5 day to get the mail or package.
In the USPS distribution center people receive incoming mail and packages and then sort them out and put them on the correct truck or vehicle in a box or other container so that they go to the correct destination.
Most mail is sorted with machines while packages may need someone to look at the address and scan it.
You cannot pick up a package from the USPS distribution center.
The USPS distribution center is not like your local post office but instead the USPS distribution center is a big building with conveyors and equipment and people in the USPS distribution center are sorting the packages and loading them onto trucks, planes etc.
When the package is at a USPS distribution center it cannot easily be found anyway so you cannot pick it up at the sorting facility.
Once the package is sorted at the USPS distribution facility it leaves that night or next morning and gets to your local post office or to another distribution facility.
When mail goes to a USPS distribution center the mail is sorted the same day and then leaves the sorting facility or distribution center that night or early morning.
When you mail something at your post office it goes to a major distribution center where the mail is then sorted and then the mail either goes out of state to another USPS distribution center closest to the address.
Or if you're in the same state the mail may leave the USPS distribution center that night or morning and then head to your local post office or a post office closest to your address where it will then be delivered.
Mail does not stay in a USPS distribution center longer than one day.
When mail goes to a USPS distribution center it leaves the same night or next morning.
The mail will not stay in the USPS distribution center longer than a day because the mail is then sorted and put onto a truck for shipping.
Thousands of packages and mail are being sorted and shipped daily and the USPS distribution centers stay open on weekends and holidays as well.