Can I Relock Bootloader Xiaomi?
Relocking the Bootloader with a Custom Rom or Wrong Regional Rom Installed Will Hard Brick Your Device! Anyways Several Ways You Can Do So: Using Miflash...
Relocking the bootloader with a custom ROM or wrong regional ROM installed will hard brick your device! Anyways several ways you can do so: Using MiFlash: Flash a stock ROM and make sure the checkbox "Clean all and lock" is checked (by default it is). The bootloader will be relocked after flashing the ROM.
Can I lock my bootloader again?
In order to re-lock the bootloader:
Connect the USB Cable to your computer. Navigate to the platform-tools directory inside the unzipped CF-Auto-Root directory. Use the following command, depending on your OS.
Can I relock bootloader after installing TWRP?
You can unlock, flash twrp, flash SuperSU, flash stock recovery, relock, then use FlashFire to flash twrp with a locked bootloader. This is how the LG V20 and the likes get twrp + root.