大门menas the gate. It‘s a noun. 门口means the entrance of the gate. Used to describe the location

The "gate" refers to the item "door", and the "door" refers to the place in front of the door.

大门--- the gate 门口--- door way/ the entrance of the gate

大门 gate 门口 entrance

大门 is the main or central door/gate as opposed to the side one.门口 is just the entrance opening.