The original Art of Problem Solving book by Richard Rusczyk, by the way, was one of the things that made me fall in love with creative math as a young student.
His Recommendations page, "Other places online to explore math", a companion to his book recommendations post. The page carries a footnote: "Some of the companies listed above have sponsored 3Blue1Brown videos in the past, but the recommendations here are not sponsored or advertisements in any way." He does not say which companies, so every commercial learning platform on the page is labelled as a disclosed relationship.
2020-06-01·3blue1brown.com ↗