The door is sprung loaded upwards in the hinge. There are about 8 lugs each side in the fuselage frame on both sides. There are corresponding metal plates on the door. When you close the door, the metal plates are above the lugs. When you secure the door with the handle, the door is lowered so the metal plates go behind the lugs.
The lugs and plates take all that pressurised load. Scary!
A 320 door
One frame lug is visible by “a” in photo by alamy.
Door metal plates are blue.