The STL and iostream are totally different libraries. STL is pretty much the foundational work of practical generic programming, while iostream is a trashfire

The STL and iostream are totally different libraries. STL is pretty much the foundational work of practical generic programming, while iostream is a trashfire of a feverdream someone had about "hey it'd be really cool if I overload operators to do I/O, wouldn't it? That would look so futuristic in code!1". The original STL implementation has even been formally verified. But you're right, modern stdlib implementations for C++ tend to be essentially obfuscated beyond any notion of readability.