It's strange how Grant Sanderson's (aka 3Blue1Brown [1]) name hasn't come up so far.
Pretty much everyone I know who claims that 3blue1brown's video helped them understand a topic... doesn't really understand that topic at all.
3blue1brown's video can be fun for topics you already know, providing a bit of new perspective, but I sometimes wonder whether giving people the feeling of understanding without the actual understanding does more harm than good in the long run.
The Borsuk-Ulam theorem is another example of something that seems obvious in hindsight but that I had never run across before.