Wifey is asking for an app or website or stuff to learn Maths with. University-level stuff; learning all the concepts and how they interact and how you apply them and things! #duckduckfedi
@star really depends on your goal: if you want a standard introduction into things you need to calculate stuff (linear algebra, calculus, bit of measure theory, a little of odes and normal stochastics), the other answers are more than sufficient. If you need a special topic, use specialised literature.
For example, for discrete mathematics (things you might need in computer science) I would recommend "Concrete Mathematics". If you are interested in numerics, i would recommend the works of Deuflhard.
Honestly, you might consider going to libgen or your nearest used book store and nabbing a free or cheap college level math book and just following along.
You could also check Harvard's or MIT's free course materials and following this syllabuses
@star thr RWTH aachen has some lectures online, they’re just a bit hard to find. It’s been a while since I last looked (used to study there before dropling out ^^)