"Learning material" for actually-advanced folks, in my experience does exist and is almost without exception books or papers. I gave up on trying to find stuff on video.
One simply has to muster up the motivation and effort to seek them out.
Honestly the best way to improve, IMO, is to just start participating and contributing.
"I hear and I forget. I see and I remember. I do and I understand."
-- Confucius
It feels like for every C# in Depth or YDKJS there's 100s of other resources strictly for the beginner. It's just hard to sift through 100s or 1000s of resources before you find something worthwhile.
after that, you have better honed long tail search queries for online.