How do I hide keypad while on a call

It is neither. It exists on the iPhone 12, as well as in iOS 14. Can you provide a screenshot that shows that it does not appear? That will also help to diagnose what your issue may be.

Once you have the keypad open, look at the screen and you should see Hide in the lower right corner. Also, if you have the phone up to your ear, then the proximity sensor will automatically turn off the screen.