Hi Betty
Thanks for posting the link.
I was lucky enough to be offered a tour once as a Sigma Pro photographer (also to the Cosina factory in Nagano where I've been 2 or 3 times). As interesting as the video is there are couple of things that I video doesn't mention that I can add that are really interesting also, at least I hope others find the info interesting!
-Sigma (and Cosina) are the only lenses lines made entirely in Japan. No other brand does this (can has vowed to return all production back to Japan at some point) (Nikon recently shut down all lens production in Japan)
-At one point, at least a pre-covid, Cosina was the only manuf. on earth to have 100% all made-in-house, even glass is made by Cosina at their glass works. No other manufacturer now makes 100% all their own glass. (Minolta did previously)
(I've been told no other makers can say this, no German lens maker can say 100% made in house of they are all 100% made in Germany now)
-The Cosina office in Nagano has large glass cases filled with all the German branded, Cosina made Zeiss lenses from 80s-90s-00s. My tour participants were shocked. There are too many lines to name, almost all modern and older Zeiss lines were made in Nagano.
-Even small parts like screws and fasteners and made-in-house at Sigma (and Cosina)! They are actually very proud of this.
-Sigma hires for life, I was told the average employee has been with them for 10+ years (I met master craftsman at Cosina that have worked for them for 30 years!) Due to this they are very slow to hire new people to boost production.
-Things like manual focus smoothness and drag finishing are still strictly done by hand at Cosina, by feel, by expert honing of lens barrel sleeves. Each lens! I have cool pictures of this somewhere. They let us handle and feel the finished parts before and after honing.
-Sigma developed their own MTF testing system for QC (not featured in the video)
-All Sigma Art and Sports lenses lenses are MTF tested individually
-Canon spends huge amounts of money on factory automation for better QC (and more profit)
-Sigma's owner invited his professional photographers (I was one for almost 15 years) a seat in product planning meetings and would ask us to give him our top 10 wish list and he would review the list in the meeting with feedback. Nikon, Canon or Sony would never do something like this. Sigma's owner is very hands-on, down-to-earth but def. not a photographer
I have lots of pictures from the factory trips and from multiple visits to the Nikon Museum in Japan on my hdds, that I've never posted. I did post one or two Cosina photos at some point on the forum.
Best,
Robert
I've found this quite interesting to watch, was surprised about the fact that Sigma allow such a look inside their factory, and that there's still a lot of manual labour involved. 
