It’s quite uncommon to have line splits for specific features. The only thing in a Tesla that might require a split is dual vs single motor. Heated seats would just be a station skip, where the worker or robot ignores cars without the feature. (Source: I used to write assembly line control software for this exact sort of thing)

It doesn’t save Tesla any money, except in marshaling. If they build a mix of lots of options then they have to track them all. With their simplified option list, cars are more interchangeable.

It also makes upselling possible, even after delivery, which is 98% of why they do it.