With rotary engines long out of production, the job of building and maintaining them has fallen to individual experts, like wizards of some kind of dark art. But there was a time when things were official, and this is what that factory assembly process looked like.
Amazingly, official as it was, Mazda was still finishing its rotary engines by hand as late as 2003, per Automotive News. What you’re seeing here is an older 13, though, according to the YouTube uploader’s description:
“Mazda Factory’s veteran Engineer, Mr. Uchitani shows how Rotary Engines are assembled. Amazing that each RE has been built by hand until 2003!”
Maybe the distinction between factory rotaries and aftermarket assemblies is not as great as one would think.