First question : If you leave the key in the ignition and open the driver's door, do you get a beep ?
As you can see, it is a bit complicated :
The door switches tell the Central Locking Pump if they are open or closed.
It then sends a tweet to the Body Control Module.
The BCM then supplies a ground to the dome light, then after a period of time it gradually removes the ground so the light seems to just fade out. ( If the dome switch is in the "auto" position. )