C++ is immensely complicated and I find it depressing that it is the industry's go to language for performance and resource constrained problems. C is also widely used (at least in embedded) but I suspect that C++ is more common for "large" code bases. The problem is that I don't see a practical replacement for production systems at the moment (yup - I know about rust and julia and Jon's language, etc...).
As to your question about how much C++ do you need to know to start using it, C++ was designed to be largely compatible with C so, if you know C, you can start using C++ for your own work and use C++ features as you learn about them and see they fit for your system (this is what Casey is doing - except in his case he knows C++ and is deliberately avoiding features he doesn't like). On the other hand, I would not let a straight C coder anywhere near an existing "modern" C++ code base if they didn't know about
exception safety and
RAII but I'm not going to comment on whether I think that style of programming is a good idea or not...