I saw many great tutorials here and there(YouTube) and they are amazing but are they really that's all needed to be a pro or above intermediate level in it like
Flutter many tutorials on YouTube to clone ui make firebase backend but are they sufficient ?
I saw few on react and node is but they seem ok that's beginner level for ya. And that was all
Other then that nothing was there. Just a bunch of projects which people make and name Instagram clone , this clone ,that clone !

So what is the way you professional guys learn old languages (Java , cpp etc ) books /docs ?
And for new languages like flutter! how do you get into them ?

I should be sophomore in CSEnginerring major