It's the standard library. People sometimes say STL to refer to the entire standard library because the names are similar and because the STL is a huge chunk of the standard library (and the non-STL parts of the standard library also tend to use templates anyway, to add to the whole confusion...)

But if you look at the repo you see the C headers like cmath, etc. that have nothing to do with the STL and are just part of the standard library.