For studios that worked on the show, Dan lists the three credited ones here https://twitter.com/DanMilano/status/1234197771093102592
And I've compiled pic related
Screenshot source links in order:>https://www.corentinhunter.net/cv
Toutenkartoon. All signs point to this studio being closed. Also incidental evidence that, yes, they do use rigging for some of the animation.>https://www.kaibouproduction.com/en/news-2017/>https://www.kaibouproduction.com/en/tv-series_en/
Kaibou(and another mention of Toutenkarton). Many studios were used for services this way, but didn't do the bulk of the animation and are uncredited. It's really hard to hunt them down and a lot of stuff is in French, so I'd be very surprised if there weren't more of them that I've been unable to find or read.>https://collider.com/glitch-techs-trailer-netflix-animated-series/
Maven Animation Studios.
That's a lot of studios.
In addition to what I did manage to find, in general, studios that started work on the show but then dropped the show later are unlikely to have any public record of doing so-- you basically have to get lucky and hope someone listed it on their portfolio, and has their portfolio online, AND that it's search indexed. The fact that there's even any public record of Toutenkartoon having worked on the show is a small miracle.
Let alone for the studios who decided not to take on the show in the first place. Your drive to not just believe shit you read on /co/ with no evidence is a good one, but some of this info can only really trickle down through the animation grapevine, either because it's not big enough news for anyone to post about it(no one's making a PR statement about their studio passing up a gig), or because people are afraid of getting blacklisted if they say too much(like implying that the low pay and high workload inflicted by inexperienced showrunners may have driven an animation studio or two into the ground).