OEM – this is a special setting responsible for unlocking the bootloader. This item can be found on Xiaomi phones with MIUI firmware. To access it, go to the following settings: “Advanced settings” → “For developers“. If the latter section is not available, go to the “About phone” menu and tap on “MIUI version” several times.
You don’t need to activate OEM unlocking just like that, as it disables all the device’s security features. But you need to enable it when you intend to unlock the bootloader. You didn’t have to do this in earlier versions of the shell. However, after the next update is released, OEM activation is required.
You can switch to enabling USB debugging when the necessary procedure is done. But it is done with the following connection of the device to the PC. A special program is installed on the PC, and the bootloader is unlocked.
