I find learning mathematical concepts from Wikipedia very difficult
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)
Some topics have excellent coverage, others are barely touched so ymmv.