One thing to note is that usually most of the door itself is usually manufactured by some external supplier and the parallel assembly line in the final assembly only mates a subassembly that contains all the complexity of the door (interior paneling, speakers, buttons, DCU, airbags, locking mechanism...) to the outer door body panel.
The space of the options for that "door module" is large enough that it usually gets ordered from the manufacturer some time after the car body gets assigned to particular order (near the start of the painting step) and these modules have to reach the final assembly line in particular time and in particular order. All is well when the whole manufacturing and logistics chain works in the same order that the car orders entered the production. But in practice there is a lot of things that can cause the ordering to become desynchronized, which leads to interesting logistical challenges (especially because if there is a need to do something out of order the order discontinuity is essentially never neatly aligned to size of the containers that the subassemblies are stored and transported in).
The same thing also applies to other subassemblies that have large option spaces, but probably the only other thing with somewhat huge option space that is not manufactured directly by the automaker are completed dashboard modules.
Also, in the traditional approach the rolling chasis and the body tend to be manufactured by separate assembly lines and then "married toggether" pretty late in the whole process. But this is usually done in the same plant, so there is less of an possibility of things getting desynchronized.