Here are the steps:

• Enable developer options
• Enable USB Debugging
• Download adb and fastboot. Example below on debian/Ubuntu

sudo apt install adb fastboot

• Go the the following page: https://developer.sony.com/develop/open-devices/get-started/unlock-bootloader/how-to-unlock-bootloader/ and get your unlock code. Follow the steps outlined there

Source(s):

1. https://developer.sony.com/develop/open-devices/get-started/unlock-bootloader/how-to-unlock-bootloader/https>https://tweakdroid.com/bootloader/sony-xperia-xz-premium-g8141/https