How did developers manually unlock bootloaders? topic

As the thread name states, how did devs unlock bootloaders before companies provided bootloader unlocking methods? I'm not looking for a tutorial, just a point in the right direction. I have the ZTE ZMAX and I can't even enter the bootloader in the first place. "Adb reboot-bootloader" simply restarts the device.

Of the above is too difficult to answer, does anyone know how to access the bootloader if the button combos and the adb commands don't work? I'm sure this isn't the first phone to face this obstacle, so I figured I would ask for some help.