[shagie]

If you look at the floor plans you can see where they're modular.

https://youtu.be/a984YzmJB4M has a video of them setting up the home and 1:41 in, you can see the two sides - one being put on the foundation, one awaiting the crane. At 4:00, you can see one with a very high peak with the roof being propped up while being assembled (I assume that it was shipped in a folded position).

When you've got that degree of modularity, then two, three or even four modules becomes feasible. The key is that the modules are assembled ahead of time.

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As to the 644 sqft - 46 x 14 = 644. The porch doesn't appear to be part of that and would add another 84 square feet if it was.