Read through https://www.learncpp.com. You'll get a complete refresher on the language, but it will be using modern C++ stuff (modern in this case means C++17).

For C++20 and newer, honestly, the only big feature that I ever use is concepts, although I do also use the new struct initialization syntax[0].

[0]: https://www.cppstories.com/2021/designated-init-cpp20/