How do I add a app-lock/face-id lock to my mobile phone apps ?

Hi, I have recently started using an iPhone XS Max. While setting up the phone, I got an option to add a face-id lock to WhatsApp, I wanted to add such a lock to other apps as well but I didn't find any option in settings to do the same. I have tried out some tutorials on youtube but none of them have helped me achieve the same.

If you know how to add such a lock to any app I want to, kindly let me know.

Thanks.