Git Bash is an MSYS2 environment that ships `bash.exe`. The rest of the Git-for-Windows executables are native Windows executables compiled with Mingw-w64 (aka GCC for Windows). Except for Bash nothing runs under POSIX simulation provided by MSYS2 runtime. MSYS2 runtime is a fork of Cygwin.

I explained the differences here: https://news.ycombinator.com/item?id=48375716