Is there an app preinstalled on PureOS that I can use, or is there a preferred method to password protecting a thumb drive for keeping files on for my Librem 14? Or is this possible?
“Disks” app is probably what you are looking for. Format your USB drive and select it to be secured with a password (gnome-disks takes advantage of luks encryption).
You can also use
cryptsetup directly from the terminal as an alternative.