A mingw deb package comes with a libstdc++-6.dll, but I can't find sources for it. A mingw-w64-v11.0.1.zip from sourceforge doesn't contain sources for stdc++.
They are part of GCC when built using MinGW-w64 as standard library.
For GCC sources see: https://gcc.gnu.org/pub/gcc/releases/
I see calls to Windows functions like _wopen() in https://fossies.org/linux/gcc/libstdc++-v3/src/filesystem/ops-common.h