"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.
Yeah I'm finding some books too, and conference talks is another decent medium but they lack depth a book or paper has.
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.
When it is videos, it's not a plethora of information on a general topic, but a very specific explanation or tutorial in a niche area, and you run into the chicken and egg problem of having to know what to look for.
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