You can see a guy following Dave Gingery's instructions to make a lathe bed here [1]

And as you say, a granite surface plate is needed. Of course, Gingery's books only claimed to set up a metalworking shop starting "from scrap" and "simple hand methods" and that "it isn’t long before the developing machines are doing much of the work to produce their own parts" [2]

Of course, to truly make a lathe from scratch, you must first create the universe.

[1] https://www.youtube.com/watch?v=zPGZg45dGXA [2] https://gingerybookstore.com/MetalWorkingShopFromScrapSeries...