To unlock the bootloader and do not need to enable USB debugging, this process is performed through Fastboot, not ADB. ADB may be needed only to reboot the device into Fastboot mode, then you cannot do without debugging enabled. And so many devices can be switched to Fastboot mode using a specific key combination (depending on the manufacturer). Another thing is that new devices will not be able to unlock the bootloader if "OEM unlock" is not enabled in the "For Developers" section.
Through Fastboot, as one of the options. In general, it is worth specifying on which device you need to do this, because different manufacturers have different methods and capabilities, there is no unification.