Those things from Homey Despot are expensive per square foot.

I have seen people buy Homey Despot's big sheds and turn them into homes. Much less expensive.

For better, faster construction i prefer post frame construction

The steel-stud (not steel frame) bungalow is placed on a foundation that you pour yourself.
Whatever is the typical foundation type in the area.
You put the sewer/septic in the ground before pouring the concrete foundation.

I do not find these prefab homes modern or amazing.
None of these work in California without serious modification for code.

Further, you REALLY need to build a much better insulated, passive solar heating house, AT MINIMUM!
R-19 in the walls, with a foam blanket around the outer sheeting or another R-9.

As someone who knows construction and design, i would almost never use a prefab home.
Most of them are just too chintzy. I could do better, for less.

Now, for something different. A real metal frame building.

You buy this metal frame, then you box it up however you want and divide up the insides however you want. With a metal roof and metal siding, it is extremely durable and great for the price.