For example, the system call in question is CreateDirectory.

BOOL WINAPI CreateDirectory(
    LPCTSTR lpPathName,
    LPSECURITY_ATTRIBUTES lpSecurityAttributes
);
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If you read down that page, you'll see that it's "implemented as CreateDirectoryW (Unicode) and CreateDirectoryA (ANSI)". CreateDirectory is really just an alias for one of these two actual KERNEL32 functions. If you look in winbase.h, you'll find

WINBASEAPI
BOOL WINAPI CreateDirectoryA(
    LPCSTR lpPathName,
    LPSECURITY_ATTRIBUTES lpSecurityAttributes
);

WINBASEAPI
BOOL WINAPI CreateDirectoryW(
    LPCWSTR lpPathName,
    LPSECURITY_ATTRIBUTES lpSecurityAttributes
);

#ifdef UNICODE
#define CreateDirectory CreateDirectoryW
#else
#define CreateDirectory CreateDirectoryA
#endif
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