As far as websites go, Khan Academy does have courses on linear algebra and differential equations I believe. However, I would recommend MIT OCW, you can find most subjects there for both undergrad and graduate courses. They have lecture notes, homework problems and some courses have the full lectures on YouTube as well.

Beyond that, your best bet is to just pick up and textbook and do some problems.

Off the top of my head I can recommend Algebra by Micheal Artin and Abstract Algebra by Dummit and Foote as a good introduction to group theory.

Topology by Jason Munkres is also a decent textbook, you should checkout r/math for more textbook recommends.