The src/ folder is divided in three subfolders:
arkemscripten/, the source for the Emscripten build, using the lib ArkReactor built by the projectarkreactor/, the compiler and the runtimearkscript/, the CLI and the REPLAll the builtin functions (and constants) are located in include/Ark/Builtins. Those can be used with the bytecode instructions BUILTIN id. Adding one will need to reference it in include/Ark/Builtins/Builtins.hpp and in src/arkreactor/Builtins/Builtins.cpp, and then implementing it accordingly under src/arkreactor/Builtins/<file>.cpp.