I remember having used nlohmann's single header JSON parser library around 2018 on a client app and found it to be pretty nifty. For REST, I had had used Boost Beast.
The fact that Beast allows you to go low level if required, is one if its strengths. And moreover, C++ is generally used in projects where such control is required.
The Beast library is now officially a part of Boost.
Beast makes REST and WebSocket programming easier in modern C++. Congrats Vinnie Falco for a job well done!!