They do not build custom cars. The option set is minimized. The build in batches.
orders are matched from inventory if possible. If not possible, the pending order info is used to decide which batch is built next.
Op this is your answer.
Tesla does not build "to order" regardless of what people like to think. They do not have any real option list, so just build cars in batches that meet those specs (like a lot of other car manufacturers). BMW, Audi, Porsche and some others let you actually order a car that is "built for you".
A tesla is just built, and matched to you when they get to the spec you ordered. Tesla only has (for model 3)
(3) Main trim options (Standard range, Long range, or Long range performance)
5 exterior colors ( black, white, blue, red, dark grey)
2 interior colors (black, or black and white)
18, 19 or 20 inch wheels.
When you place an order (with deposit) they attempt to match you to a car that is in your area if possible. If not possible, they put you in the queue for the next time they are making that trim (so yes, fremont still builds the spec car you "ordered" if you cancel your order.. they just match it to someone else, or ship it to an area where there is demand for that spec to sell to someone else).
If two people order the same spec with 1 "in inventory" it would go to whoever is closer to said car logistically... although there are not usually a lot of brand new, never been delivered before, cars in inventory. They are normally in transit "somewhere".
It someone orders "not to be delivered before XXX date" they put you on a "do not deliver" list so you are not put in the queue for a car, or matched with a car, but cars with that same spec are still built in the batch when its their time, they just dont match it to you, they match it to someone else.
Its more like ordering a box of frosted flakes from amazon. If you dont order it, its not like they dont make the box anyway. In no case are they really "building a car with "your" specs. They are building cars of various specs, and when they get to the batch that includes yours they match you with it.
Every car has all the hardware for FSD so its really color of car, and wheels.