this is the POSIX C runtime, the reskit/BSD userland, the X11R6.9 / Motif 2.2.2 / CDE client stack, and the runtime data slipstreamed onto the ReactOS CD.

The core subsystem itself lives in the ReactOS source tree and stays there:

Clone this next to the ReactOS source tree so the two are siblings :3 :

somewhere/
├── reactos/         (the ReactOS tree with subsystems/posix)
└── reactos_posix/   (this repo)

reactos/subsystems/posix/CMakeLists.txt detects ../reactos_posix at configure time and add_subdirectory()s it, so everything here is configured and built as part of the normal ReactOS build and lands on the bootcd ISO.

Userland targets are named posix_<name> (emitting <name>.exe) to avoid colliding with ReactOS Win32 targets; sh keeps its plain name.