I’m not sure how all the car companies do it. Most factories I was in were pretty old, so they were designed decades ago before a lot of the modern manufacturing techniques. We knew the cost per hour of running the factory. We knew the cost of all the raw materials and parts that go into the car. It’s pretty easy to calculate the line rate required to be profitable. That turns into each team’s time goal for adding their parts to the vehicle. By the time we got to PVT, we would built the cars on the actual line mixed in with the existing model. Every 5-10 cars would be a prototype. At that point, you better be ready to go at line rate, because no one wants to be the person to cause the line to stop.
I know Toyota uses the same manufacturing process for every vehicle. They design all vehicles, so they are built in the same order. That’s why they can build a truck, SUV, and car on the same assembly line. It’s pretty amazing. Their US factories are pretty new, so they take full advantage of the modern approach to manufacturing. Honda does the same.
Most people are doing the same thing to increase line rate and reduce costs. They are pushing suppliers to provide modules vs components. That way the sub assembly labor is in a non-UAW factory. They used to assembly the dash in the auto factory. Now they come in fully assembled. Production is planned enough in advance they come in exact order for the assembly line. The operator verifies the dash and body match by scanning a bar code on both.