Buyer tells dealer what they want.
Dealer looks at all build configurations scheduled at the plant that are currently available and shown on their internal system.
When dealer finds a close enough match, they use one of their allocations to “claim” that build.
Dealer then has a specific build and build date to share back with buyer, usually also a temporary VIN.
Once that date is reached, the van gets built and an actual VIN is assigned.
It’s then put on a train/truck heading in the direction of the dealer to whom it’s allocated.
Van stops at a centralized hub or port and has accessories installed (sometimes these port installed items can be changed, but nothing from the factory can). Then it gets put on a truck and dropped off in dealer lot.
Dealer calls customer to come and get it and transfer payment.
Buyer drives home happy and starts playing with all the bells and whistles and opens an account here to ask why the 3rd row doesn’t lay flat and why AWD didn’t come with 20” wheels.
No “customization” needed.