LlamaBarn is the MacOS app, not the HTTP API server, which is "llama-server".
On non-Apple PCs, "llama-server" is what you use, and you can connect to it either with a browser or with an application compatible with the OpenAI API.
Perhaps using "llama-server" as the name of the project would have been less confusing for newbies than "llama.cpp".
I confess that when I first heard about "llama.cpp" I also thought that it is just a library and that I have to write my own program in order to implement a complete LLM inference backend.