with xiaomi phones, unlocking the bootloader is not as simple as running fastboot oem unlock or such from a PC. The reason for that is that manufacturers get to decide how to implement those fastboot commands, and xiaomi decided to restrict it quite a lot.

my last experience was that to toggle on the switch on the phone that makes unlocking possible, you need

• a xiaomi account
• a SIM card in the phone that can send SMS
• unfiltered internet connection (more precisely it will want to connect to a specific xiaomi domain)

then, you need to install the xiaomi unlocker tool on a windows PC, log in with the same xiaomi account as on the phone, connect the phone over USB, start the unlock process, wait like 2 weeks, and finish the unlock process with the same tool.

the unlock tool most probably just retrieves a secret code from xiaomi and passes that to fastboot, but you can’t really get around it afaik, unless you want to risk rooting it through a vulnerability, which will still leave it locked