I agree with the GP that you really need to work through a lot of problems to fully understand a mathematical subject. But there are a couple cases where I did have real, lasting moments of insight, and the 3B1B linear algebra series was one of them. Hearing it stated directly that the columns of a transformation matrix are the coordinates of the basis vectors after the transformation (a fact that I had somehow missed across multiple linear algebra classes) turned on a light bulb in my head.

The Borsuk-Ulam theorem is another example of something that seems obvious in hindsight but that I had never run across before.