replace removes the existing fragment and adds a new fragment. This means when you press back button the fragment that got replaced will be created with its onCreateView being invoked.

add retains the existing fragments and adds a new fragment that means existing fragment will be active and they wont be in ‘paused’ state hence when a back button is pressed onCreateView is not called for the existing fragment(the fragment which was there before new fragment was added).

In terms of fragment’s life cycle events onPause, onResume, onCreateView and other life cycle events will be invoked in case of replace but they wont be invoked in case of add.