I'm from a STEM background and already know category theory, and I already know the basic syntax of Haskell - where can I go to see what the people proficient at the language are doing?
I mainly program in C++, and there are literally tonnes of advanced projects to find on github using well-written C++ to get into, not to mention all the videos, podcasts, discord communities and books on C++ best practices.
The best option I've found for Haskell so far has been to look at the standard library implementation for different functions, but what else can I use/do you use for learning intermediate/advanced Haskell?
Along the lines of perusing the standard library like you mentioned, depending on what problem domains you are more familiar with, you could look at libraries for those domains as examples.
As far as books go, people say good things about Haskell Programming from First Principles: