Well these centralised resources are books. There are high quality (almost standard) books for most subjects. Want to learn Analysis at a Graduate level? Pick one of the popular books Rudin (for example) and work through it. Same goes for Quantum Field Theory (Peskin & Schroeder, Zee, Srednicki,...) and tons of other subjects.

The lectures are often only a guided tour through one of those books (at least at the undergraduate and early graduate level). In many ways lectures are still la huge advantage of course because most of these books are too long to be useful (I still have some guilt when I think about how little I absorbed of the 500+ pages experimental condensed matter book we used). Often the exercises then cover things that were not explicitly explained in the lecture but can be worked out with the help of the book.