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.

Changing Bootloaders topic

The CM12 release seems to kickback an error if the bootloader and recovery aren't properly aligned. But I haven't been able to find a way to specifically change the bootloader, I suspect its going on behind the scenes. I've got custom recovery (TWRP, and i have tried several versions) and have flashed a wide variety of ROMs. I also just "rerooted" my device with effortless stuff because I was getting signal issues. Even in this rerooting process I didn't seem to directly change the bootloader.

What bootloader/recovery combo can i flash to get CM12 working?