Xiaomi (or Mi) is a Chinese manufacturer of electronic products.
They produce a wide range of products, including smartphones, home appliances, and even electric scooters.
Xiaomi produces relatively affordable smartphones with decent specifications.
Unlocking the bootloader
It is possible to unlock the bootloader and install a custom ROM on most of their phones, but the process is not as easy as some other manufacturers.
You need to register a Mi account, which requires a phone number. After you register, you need to use their unlock tool. This cannot be done immediately, you need to wait for some number of days. Also the official unlock tool does not work on Linux.
After the days have passed, the bootloader can be unlocked, but it will wipe all data on the phone. And you cannot backup all the data, because you need root access to do that, but you cannot root the phone without unlocking the bootloader.