[unwind]

Uh, I'm not in the industry at all, but obviously (?) the concept of prefabricated (modular) houses exists, and has done so for many years.

Perhaps this is a regional/national thing (I sometimes get the feeling that "prefab" is a condescending term in the US sometimes). For reference, here's a page showing the selection from one of Sweden's main house manufacturers [1]. It's in Swedish, but I just wanted to share the pictures. Also it's hard to say exactly what proportion of a house is pre-built, but I expect the majority of the structural things (walls, floors, roof) to be.

[1]: https://www.eksjohus.se/husmodeller/

[twic]

I believe prefab is also more common in Germany. I became aware of this when a UK TV show showed someone having a a Huf Haus installed:

https://en.wikipedia.org/wiki/Huf_Haus

According to this random web page, there are quite a few manufacturers:

https://medium.com/blue-future-partners/our-9-favourite-germ...