I have been training in boxing and kickboxing for a couple of years. This year, I also started trying other stuff like Krav Maga and Brazilian BJJ.
Here are a few resources that I found usefull.
YouTube
Joe Valtellini found it usefull when i tried to do some harder sparrings
Skillr Boxing Found understandign of useful boxing concepts that are not discussed in hobby gym.
BJJ fundamentals helpful introduction to core Brazilian Jiu-Jitsu concepts.