Qt? Because there are a lot of companies hiring specifically remote for Qt? It's not my impression that in general, the majority of jobs using C++ expect/leverage Qt.

There's tons of embedded work using C/C++, but indeed remote work is harder to find there.

because all C++ projects use Qt?? what?

I know Qt (have an open-source project based on it https://github.com/a-rodin/qstardict), but don't want to work with GUI. I think I would like some libraries or applications that run on server and so some complicated jobs that require high performance.

I'm giving this one an upvote. You certainly don't need to know Qt, but when I recently asked a remote / nomad developer about finding C/C++ work, he also recommended learning Qt. He said that while he didn't get C++ leads very often in general, he did see Qt leads coming through to him semi-regularly.