This is the original TWRP Recovery for Mi Mix device. TWRP is a masterpiece of Team Win dev team and it is the most popular custom recovery app for many devices especially those running MIUI ROM. Featuring touchscreen-enabled interface, TWRP has many capabilities including to flash custom ROM, SuperSU (root), Gapps, and many others system-only apps. On top of that, it can also create and restore full device backup (nandroid backup) easily.
A custom recovery is used for installing custom software on your device. This custom software can include smaller modifications like rooting your device or even replacing the firmware of the device with a completely custom “ROM”.
Commonly, TWRP can be installed instantly via Flashify app on a rooted device. However, if your device has never been rooted, downloading TWRP image file (.img) and flashing it through a fastboot interface is the only way. It can be quite tricky due to Mi Mix has db-verity mechanism. Therefore if the device is still using stock kernel then once TWRP flashed and you opted to swipe for “allowing system modification” will prevent the device to boot properly.