As I've pointed out before, if you want a metal box building, there are good components for that. Butler Buildings and their competitors make them in the US, and they're everywhere in rural America. You can get insulated walls, windows, and doors. They're not pretty, but they're weathertight and strong.

One of the companies in that industry, General Steel, now offers house kits.[1] The main problem is that many municipalities don't like houses that look like industrial buildings.

[1] https://gensteel.com/steel-building-kits/houses