How can I stop my iPhone from locking unexpectedly?

If you routinely carry your phone in your pocket (not a good idea for a ~$1,000 piece of delicate electronics unless it is in a protective case) you need to do 2 things:

(1)Go to Settings/FaceID & Passcode and turn off Raise to Wake.

(2)Put the phone in your pocket with the BACK against your body so skin contact doesn’t activate it. This may seem counterintuitive, but it is more important to protect the back than the front; a screen can be easily and quickly replaced, but if the back glass breaks it is a major repair to fix it, and in some models the whole phone needs to be replaced.