I’ll be the first to say that I have not gone in depth at all with regard to this site. That being said, I have had countless conversations with very bright students who have referenced this as a resource they love. In one of the discussions with a student who was in my Linear Algebra and Calculus III course, the topic of Quaternions came up. I was introduced to this in Space Vehicle Dynamics with Dr. Bong Wei (who literally wrote “the/a” book on it, https://arc.aiaa.org/doi/book/10.2514/4.103803, side Trivia, his niece is Michelle Wei who tours on the women’s LPGA).
So i was intrigued that this undergraduate, albeit a bright sophomore, was learning up on topics that are usually Senior level / Graduate work. I checked out a few videos, and this guy does a very fair job. It is geared more towards preliminary understanding as opposed to example driven (like Paul’s where you have 3-5+ examples for each topic), however, I do believe he does a great job in his explanations of advanced topics, not just basic Calculus I.