> How exactly does your debugger know whether the compiled code it stepped into came from C++ or Fortran source?

Executables with debug symbols contain the names of the source files it was built from. Your debugger understands the debug symbols, or you can use tools like `addr2line` to find the source file and line number of an instruction in an executable.

Debugger does not need to understand the source language. It's possible to cross language boundaries in just vanilla GDB for example.