Have you tried simple Wikipedia? I find it way easier to conceptualization the topics there and then move onto the main Wikipedia to get more detail, as needed.
I also agree, and would further that any relatively technically involved concept tends to contain the most complicated form of that topic, for example the wikipedia page on lift goes into some pretty intense detail.
This is the natural result for a website written by users (those with lots of knowledge write it for others who have lots of knowledge). Even simple wikipedia is not immune to this, due to explaining the concept theoretically instead of taking example problems.
I do not mean to say that I think wikipedia is bad or that others do better. I'm not even sure such a problem is solvable at all (making it useful for both beginners and experts)
On the other hand, articles written by experts with full detail is much more useful to those already well versed in a given field. To use myself as an example: I would look elsewhere for computing or linguistics information if the articles were all written for beginners.
I don't disagree with you - the articles are not at all useless in topics for which I am well versed. Like I say, the issue that is perhaps unsolvable is some intuative way to have the article useful to everyone at once while not bogging them down in useless info
https://en.wikipedia.org/wiki/Derivative
https://simple.wikipedia.org/wiki/Derivative_(mathematics)