You will never need to register your face or fingerprint again when using this feature. WhatsApp will rely on system-based biometric authentication.
Here's how to enable it:
For Android devices
Open WhatsApp.
Go to Settings> Account> Privacy> Fingerprint Lock.
Activate unlocking by exchanging fingerprints.
When this option is enabled, you will be prompted to authenticate with Face ID every time you open WhatsApp. The steps below describe how to enable this with Face ID. If you have Touch ID on your phone, the procedure will be the same.
For iOS
Open WhatsApp.
Go to Settings> Account> Privacy> Screen Lock.
Operation requires Face ID.