Most (all?) top universities have free educational content, often including entire courses, available. For instance here [1] is MIT's open courseware site where

Most (all?) top universities have free educational content, often including entire courses, available. For instance here [1] is MIT's open courseware site where you can download all the required media, including lecture video/notes/problem sets/exams/etc, for courses - completely for free. Things like this are generally going to be orders of magnitude better than any YouTube video. [1] - https://ocw.mit.edu/ https://ocw.mit.edu/