Modeling car doors and fitting to the body

shift drag border back a little

same thing with the door

weld 3 vertices each corner of door opening

hide unselected polygons

shift clone ALL polygons to clone to element

flip the new door element

shift drag back that border

vertex weld

detaching to an object will allow you to detect 2 borders on the door and then use bridge

.

re-attach and apply turbosmooth

chamfer each edge some small amount

extrude or shift drag border edges in

make the same small chamfer

you might need to do a little modeling to get the door form of not flatten against the body ... here we pulled out the inner vertices...

Even after the basic work there will be revising of the mesh for the plates to mold together perfectly