Javac itself is written in Java but it is provided as an executable file when we download the JDK. What tools are used to compile the Java sources of Javac to an executable?
A JDK, called boot JDK which is usually the N-1 version of the JDK you're building, plus C/C++ compilers. At least for OpenJDK, the supported toolchains by Oracle are gcc on Linux, clang on macOS, and msvc on Windows. Additionally, SAP maintains the AIX build using XLC.