This doc contains a list of youtube channels and other resources that people have found useful for learning more about firmware and embedded systems in general. Feel free to add your own section and drop some of your favourite resources!
Navtaj:
Ben Eater: http://www.youtube.com/@BenEater
Andrew Morton (especially mte 241 playlist): http://www.youtube.com/@andrewmorton3272
Jeff Zarnett: http://www.youtube.com/@jzarnett
Low Level Learning (useful for learning more about programming and C): http://www.youtube.com/@LowLevelLearning
MATLAB (not as related to firmware but cool stuff): https://www.youtube.com/@MATLAB/videos
NetworkChuck: http://www.youtube.com/@NetworkChuck