Well, you just kinda answered your question. There is STL, Qt and Boost. Like all other languages. They do the same thing but in different capacities, you should select based on your needs. I think std::string can be supplemented for Unicode support, I am gonna say the fmt library was the supplement but I feel like I am wrong.

I used std::string and QString extensively for 4-5 years and find QString inexplicably bloated and un-intuitive for my use cases.