Here are some of the best YouTube channels to learn more about modern VFX and the theories behind them:
Peter McKinnon https://www.youtube.com/user/petermckinnon24
Peter McKinnon has a lot of YouTube tutorials detailing the basics behind Adobe After Effects. It's a good place to start to learn more about the overall theories and structures in film editing.
VideoCopilot has a lot of tutorials for cool VFX such as lightsabers, masks, and other 3D stuff. They do sell their own plug-ins but their tutorials do have a lot of substance despite the marketing.
Film Learnin https://www.youtube.com/user/lonearcherfilms
Film Learnin has a lot of tutorials aimed at those who want to copy or learn how to make pop culture VFXs. For example, he has tutorials for almost every Marvel and DC VFXs such as Flash's lightning streaks, Dr. Strange's shield effect, Thor's hammer catch, and more.
Corridor Crew https://www.youtube.com/user/samandniko
Corridor Crew does not have much tutorials but what's helpful is how they discuss about CGI and VFX in the industry. Viewers will learn more insights on how certain VFXs work or why they don't work for certain audiences.
KriscoArt has tutorials for almost all Adobe Creative Cloud programs. The channel is useful for those want to be well-rounded as an VFX artist.