I am a trivial high school student who loves maths. Recently I found some maths-related YouTube channels. Although they cover a broad range of mathematics, many of them are about integration. So I mainly watched videos about integration.
I found such integrations very difficult or impossible to solve with the scope of the high school maths curriculum. We learn some basic substitution and integration by parts, however, they are insufficient.
For most 'interesting' questions on YouTube, it is not necessary to know the university level of calculus, but the problems require either patience or ingenuity. They often involve several substitutions (which make the calculation very long and complicated) or require creative thoughts that people cannot easily come up with. Challenging questions give me a kind of enthusiasm and encouragement at first, but after 30 mins or so, I feel depressed being unable to solve them and realising my lack of talent.
I fully acknowledge that these YouTubers (or anybody else teaching mathematics and doing such interesting questions) are highly enthusiastic in maths so that they do not hesitate to put a lot of effort learning and practising, and that they are talented and knowledgeable. However, I wonder how they can think so deeply as such, and how they can use such sophisticated arts of techniques to approach to problems.