YouTube is actually one of my very first places to go for new concepts. For example, one of the recent Advent of Code puzzles was solvable with "coordinate compression," a technique I had never heard of before. I didn't find much, and what I did find wasn't an especially high-quality presentation, but it did teach me a magical new concept that helped me to finish the puzzle.

I have also benefited so much from MIT OCW lectures. The quality of their teaching is so high that it showed me that when my children go to college, it will be worth it to send them to a much more expensive elite school.