A bit more like Kevin Powell, who publishes lots of free CSS videos [0], has some good basic CSS Training Courses for free, but charges money for the advanced ones [1]. His videos are not only available on his site, but they're also on Front End Masters, which is a well known training site.
While I can't speak specifically to this person's earnings, I suspect you're overestimating just how much money average YouTubers make from owned platforms when they try to push their audiences to them.
It's very difficult to do this in a way that produces sustainable, durable revenue, and I suspect that people doing technical educational content are finding it increasingly difficult in the age of AI.
Trends for his YouTube channel look pretty much like what I expected given the general trends in the marketplace.
[0] https://www.youtube.com/@KevinPowell
[1] https://www.kevinpowell.co/courses/