From my own experience, they're probably flashing the RPi4's EEPROM with an updated bootloader and config to enable netboot. Here's the official documentation on it: https://www.raspberrypi.org/documentation/computers/raspberr...
> I flashed each of the Raspberry Pis using the same SD card, then I could close the case and take a note of each MAC address and serial number for later on.
But if flashing SD cards, then why do we need network boot? Or, if we have network boot, why do we need to use SD cards?