AoPS Alcumus.

Beyond that, OCW is pretty good. Find homework assignments from good courses.

As a footnote, the linked-to article is mostly nonsense. If you want to learn math:

- Work on good problems

- Once you've struggled for a bit, go back and look up how to do them

Yes, this includes problems you don't have prerequisites for and have no chance of solving. You shouldn't start by watching videos or explainers until you've struggled. There's a pile of science behind this.