How does a mail truck work?

Any mailmen or mailwomen out there?? Do you sort through letters and packages yourself on the truck? Are they pre-sorted before you drive your route? How often do you go back to the package office a day, or do you have all the mail and packages for the day already in the truck?

I cannot wrap my head around how millions of packages AND letters are sent around the US and the world every day and somehow they all get to where they are going on time!!!! It is SO IMPRESSIVE to me!!