None in the default OS, it uses existing C++ build tools to build the OS, but the OS itself (inc. Kernel, GUI & default Apps) was built from scratch. Including Font Editor (https://github.com/SerenityOS/serenity/tree/master/Userland/...), rendering and Crypto impl (https://github.com/SerenityOS/serenity/tree/master/Userland/...).
It does maintain a large number of ports you can install as add-ons to be able to use existing software, tools, languages & games, etc in SerenityOS.