A lot of comments here are talking about starting a civilization from scratch or ancient practices. Those are interesting topics but be aware that's not really what the "Primative Technology" channel is about. He's really just a hobbiest seeing what he can do from scratch without modern tools. He's not practicing for a collapse or reconstructing the past.
Starting from 1997, they picked a year 1229 as the simulated start date, then worked to build a castle using, for the most part, only extraction, refinement, and building practices from that year. They may adopt new technologies as they were found in 1230, 1231, etc. as the years progress.
Apparently some of the folks restoring Notre Dame went to apprentice there for a little while to learn some of the woodworking techniques needed for their work on the cathedral.
A corollary in the digital domain is the bootstraping required for modern elf/linux based OSes: https://bootstrappable.org/projects.html (aka moving gcc to c++ was a huge mistake).
