“Mathematics is not about retention of content.”
I guess you’ve never heard of Saxon, Kumon and DISTAR? Mastering the multiplication tables? You’re not going to know what 8x7 is just by watching a video about multiplication or doing a couple of random problems before going on to the next topic.
Math is based on a foundation of prior discovery. That prior discovery need not be memorized (unless you are a stupid HS teacher who is still trying to earn his pension).