Yes the USPS US mail does get picked up on Saturday.
If you mail something on Saturday by 4:00 PM the mail will be picked up and then be put onto a truck and sent to the sorting facility.
However if you mail your letter or package after 4:00 PM then the package or letter will usually stay at the post office until the following Monday.
The USPS lists the mail pickup time on their mail boxes and some post offices stop picking up mail inside the post officer before 4:00 PM on Saturdays but some continue to pick up the mail.
On the outdoor blue USPS mail boxes they pick up the mail usually by 4:00 PM and sometimes a bit later depending on the post office or box location.