Assume the door is open parallel to the a/c fuse'. The interior door handle, which moves vertically, is in its up position about 20 degs from the vertical.
You now pull the door towards its frame, as the door enters the frame a lever on the door hits a plate in the door frame, this unlocks the lifting mechanism that has been holding the door in its current position. At the same time two lifting arms, one on each side of the door enter their guide channels in the door frame. The door now in the 'hole' but higher than its closed position. You push the handle down, this lowers the door into the fuselage profile by rotating the lifting arms in the channels. The last inch or two of handle movement moves the mechanism of rods & tubes that operate the lifting arms into an over centre postion. At the same time at the top of the door two locking latches contact their mating halfs in the door frame. This mechanism also goes over centre. The door is now locked shut.
An Airbus main pax door is 92 in x 52 in which gives a total area of 4784 sq inches. Now lets be conservative and pressurise the a/c to a diff press of 5 psi (it can be up to 8 psi). This means at that diff press there is 10.7 tons pressing on the door. So to open it in flight you would have overcome the 10.7 tons.The pressurisation loads are taken by about eight stops on each side of the door that butt up against stops on the door frame when the door is lowered into position.
Incidentally the door is sealed by a hollow rubber seal around the edge of the outer door skin. The seal has holes in it on its inboard side that allow cabin air in and expand the seal thus sealing the gap.
Having dug out my manufactures course notes ( the last time I worked an A300 in 1984) I found I was wrong about the klaxon. That is for when you try to open the door with the slide armed. However there is a warning light on the door that operates when the diff press is greater than 0.4 psi, the engines are not running and the door is not armed for slide operation.
I hope that answers your questions