What's the expert functions on Programming languages?

Question

I've watched people on Internet that write about Sololearn and Many opinions go to a central idea... It's "Sololearn has a basic level" and I've searched for some other courses on Internet and almost always has the same content like Sololearn So... I'd like to know What is the máster área of Programming languages? for example: C++, what's the content that I need to know from C++(considered as an Advanced level) which It's missing here(SL)?
Pd: My question is about Any Programming language, C++ is just a Sample :) .