WhatsApp
If you still need to use WhatsApp to keep in touch with friends and family, there is an option to add a screen lock on iOS and on Android, although you’ll only be able to do this with either Face ID or Fingerprint ID, which has its drawbacks.

iOS

Go to Settings > Account > Privacy
Tap on Screen Lock
Toggle Require Face ID or Touch ID on
Set the time duration for when to require Face ID or Touch ID
Android

Go to Settings by tapping the three vertical dots on the top right corner
Tap on Account > Privacy > Fingerprint lock
Toggle on Unlock with fingerprint
Verify your fingerprint to enable the screen lock

Sources : ExpressVPN

https://www.pcmag.com/how-to/20-essential-whatsapp-tips-for-chat-fans