Go to settings select Passwords & Security -Fingerprint Unlock after that users need to put password /pin/pattern once this is done place finger on fingerprint scanner till fingerprint is added. After that lock the device and Unlock it using a fingerprint.
Five fingerprints can be added. Fingerprint Scanner on Redmi Note 9 can be used to Unlock the device, Unlock Apps using Fingerprint Scanner. There is no option to Capture selfies using the fingerprint scanner. Fingerprint Scanner works fast.
To enable Face Unlock Go to Settings- Passwords & Security -Face Unlock. It works well under good lighting conditions.