Huawei bootloader: what is "FB Lock" and how to unlock it?
There are two types of bootloader locks in Huawei devices: FB Lock and USER Lock.

USER Lock restricts the standard partitions like system, data, recovery, kernel etc, but not the critical bootloader-partitions which I don't remember the names of. (So you can't edit them even if you perform a USER Unlock) The USER Lock is what requires the bootloader unlock code to unlock. Also, there are some hidden hardware commands which are used for serious debugging like JTAG, that cannot be accessed with a USER Unlock.

FB Lock is used for protecting these restricted functions. A FB Unlock works as a key for all of Fastboot. (So if you perform a FB Unlock then you can unlock everything USER Unlock does, and the other restricted stuff) FB Lock is intended to be used by Huawei personnel for repairing. It is NOT supposed to be unlocked by users, but it is possible anyway, as I discovered after quite some research, but it doesn't work on very new firmware versions like EMUI 8.2 or July/August Security patch. It has been used for years by paid software for performing FRP Unlock, full system restoring, and it can even be used for obtaining a bootloader code for USER Unlock. A FB Unlock is usually temporary.

The method of FB Unlock has been kept behind cover for quite some time now, but I saw recently that it has been leaked on XDA. You can check it out here. That guide also covers a method for extracting the NVME partition for a permanent FB  Unlock. If you don't want a permanent FB Unlock, but would like just a normal USER Unlock, you can skip doing steps after 3, but instead when you boot into TWRP, you can use: 

su -c "grep -m1 -aoE 'WVLOCK.{14}[0-9]{16}' /dev/block/mmcblk0p7 |grep -aoE '[0-9]{16}'"


That will output the bootloader unlock code which you can use for standard USER Unlock. Good Luck!