Curious about kanokarob's method to generate random paths

Hi,
I am trying to figure out how <@205037619065782272> does his magic to generate his structures with a random path to the end portal room. In one of his videos, he said that he generates the jigsaws first and then generates the structure on top, but I have a hard time seeing how that works without overwriting the jigsaws with the structure. Can someone explain how it works?