I feel like posting GDC video is too obvious. i watch a ton when I have time, if you don't already, do it, they're just amazing. one of my favorite video is the Diablo 2 post-mortem, where the diablo 1 and 2 director relates the experience of the birth of Blizzard North, developping both games and creating the action RPG genre, bet a lot of you saw it but here it is
View: https://www.youtube.com/watch?v=VscdPA6sUkc
Extra credit, obviously. Best channel NA.
Didn't see game maker's toolkit mentionned. It's an amazing channel in the same vein of EC but much more focused on pure game design, with high production values.
https://www.youtube.com/user/McBacon1337/videos
Gnomon is one of the top VFX school, they teach everything from concept art to all 3d skills to animation. Their channel has amazing dev interviews, similar to GDC but more geared towards game art and vfx.
https://www.youtube.com/user/gnomonschool/videos
One of my favorite video from them is the interview of Glauco Longhi, Brazilian character artist at Naughty Dog, and his inspiring journey from learning the human anatomy while making movie props and costumes in Brazil to getting hired in a world class game studio.
View: https://www.youtube.com/watch?v=2uhOEFNSMW8
If you have a bunch of money to spend on video tutorials, the gnomon workshop website is great. They have great tutorials, from legends of the industry. Artisticaly I don't always like what's being done there (a lot of the typical photobasher concept artists come from them) but they have some videos from people like Syd Mead explaining how he imagines the futur through his paintings (he entirely designed and did many mate paintings for Blade Runner, was a concept artist on Alien as well...and made a CSGO skin?
https://csgostash.com/skin/800/AUG-Syd-Mead https://en.wikipedia.org/wiki/Syd_Mead LOL), disney animators, Ubisoft art directors...and they have a bunch of environnemental design and level design tutorials.
https://www.thegnomonworkshop.com/
Can't get over the fact a 83 yo legendary concept designer actually made a CSGO skin.