Speculation: People think that Tesla build in batches. This probably isn’t too speculative because, from a manufacturing process, it only makes sense to build in batches. Taken to the limit, if they “build to order” they would be getting, say, 300 orders per day with multiple different specs and building one, then the other, then another, in that same day, just wouldn’t make sense.
Not really; This does make good business sense and this is how most modern car production lines work. It all starts from the initial order, which triggers the production line slot, and automatically triggers orders for the parts from (Tesla's) suppliers when they need them (ie some bits are ordered that day, some maybe 4 days later, some 7 days later etc..); which then get delivered "Just in Time" to the assembly line. Then they get bolted on as the base car passes through the line. Which in the case of the Model Y, the variation list is tiny; seats, wheels, trim. All of which they will have a buffer stock of anyway to prevent halting the production line. This is the Kanban system which has been used in manufacturing for decades (They use the same methodology in McDonalds !)
Also, the MY cars are all exactly the same! The only difference is software activation and the initial body colour. Everything else is just bolted on; wheels, seats, wood/white trim. I bet even the LHD and RHD cars are exactly the same just with the steering wheel bolted opposite side underneath Tesla's symmetrical dash (unlike most manufacturers, where they have a completely different Asymetrical LHD/RHD dash and different parts, air ducts, wiring harness, switches, brackets, knobs, mirrors, labelling etc. !)
If you have an intelligent paint booth (or more than one) on your production line, you can have multi coloured cars coming off the production line all muddled up with each other 24 hours a day (and most do). Its just not an issue.
The main way manufacturers optimise efficiency (and save money), is to design for manufacture and reduce complexity which means holding less parts inventory by having less options, hence sharing a significant number of parts with the model 3, only 2 interior options, only one propulsion unit, only 2 wheel options and producing only one variant (LR) etc. Its pretty cunning stuff !
Maybe Tesla do secretly manufacture to order? maybe they don't? We'll probably never know. Considering how much into tech Elon is, and how long this methodology has been around in the automotive industry I'd be extremely surprised, if the orders didn't trigger the parts. What happens after that, well.... people are allowed to change their orders, change the colour, change their wheels, so perhaps this could be what triggers the "un-matching/matching" process. It's all a bit bizarre this matching you to a car while its on the boat thing. Why would you just guess and make lots of cars in lots of different spec, when you know who's ordered what at the very beginning??? It could all be just a ruse to create artificial scarcity. Who knows?
Anyway, this all works fine, if you don't have a pandemic and your production line isn't shut down, and your suppliers aren't shut down, there isn't a chip shortage and you don't have to ship your product half way around the world on a boat. Otherwise it'll work perfectly. Once the German GF is up and running, I'm sure it'll be 100% smoother!
But I just don't get black wheels.
