Ask HN: Sources to learn powerful everyday useful mathematical concepts

fam I already have engineering degree and graduation level mathematical knowledge. But generally maths can get detached from application. i want to know sources from which we can learn powerful and useful mathematical concepts like compound interest used extensively in finance and i use it very much for my personal finance. corollary of compound interest can be finding present and future value I want to learn of amazing mathematical concepts that you use and where to learn more of them.