C++ is not a common choice for web backend. The common options are Javascript, Go, C#, and Rust. It is certainly possible to build a web server in C++, but there are two important problems with it.
- There is no dominant library/framework for it, although there are many non-dominant ones. In most cases, those who use C++ to build a server end up building it from scratch.
- Security is very difficult when building servers, and there are a tremendous number of ways to accidentally introduce security vulnerabilities in C++.
It is only common to build a server in C++ for high frequency trading, but this requires very specialized knowledge. There are also very few job opportunities, although they pay in the millions.