Hello,
I use two RPI3, one for each printer, and I have access to the USB ports.
When I make a rpi-clone, I clone my sd card through this port.
But how can I boot through this port.
The 3B+ supports it out of the box, I think you just have to set it to prefer usb in raspi-config over SD card. More details can be found on this page. Keep in mind, any reference to /boot/firmware/config.txt, that file will actually exist at /boot/config.txt.
https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#usb-mass-storage-boot