You’d know someone that could do work, or you would do it yourself. As for materials the supply chain used to be much more local, even today in rural areas its likely possible to buy lumber (like a 2×4) from someone that milled it in their yard from locally cut wood. Modern supply chains lower the cost (and maybe the price) but the ability to produce locally still exists.
Also Sears used to sell kits for an entire house, you could literally order a house from a magazine. They’d send you all the materials and the instructions to assemble/build. [https://en.wikipedia.org/wiki/Sears_Modern_Homes](https://en.wikipedia.org/wiki/Sears_Modern_Homes)
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