libc is a UNIX concept, as the OS API surface, as described by POSIX.
On any other no-UNIX derived OS, it is the C compiler standard library, covering only the ISO C specifies.
All the remaining OS services are exposed by other libraries, which in Windows case, the bare minimum is user, kernel and gdi dlls for the Win32 personality.
Systems like IBM i, IBM z, ClearPath MCP, .... also have similar set of libraries, as do non-POSIX RTOS for embedded.