How to detect macOS version?

Question

I am trying to build a lib for macOS and Linux. The reason I am writing is to know how to detect macOS 11+ and Linux in a header file.

I have the structure on cmake, and it's building, but the os.h header is not building.

How can I detect with #ifdefs whether my code is on macOS Big Sur+ vs Red Hat? I am working on both sides.

I tried to use #ifdefs without success. This lib made with cmake is a proprietary code. I read a lot here about the subject without success. Can you help me?

My os.h:

#ifndef OS_H
#define OS_H

#include <iostream>
#include <sys/stat.h>
#include <sys/types.h>

#include <stdlib.h>
#include <map>
#include <string>
#include <sstream>
#include <vector>
#include <cstring>
#include <vector>
#include <fstream>
#include <ctype.h>

// os detection version
#ifdef Q_OS_LINUX
  std::cout << "Linux version";
#elif defined(Q_OS_CYGWIN)
  std::cout << "Cygwin version";
#else
#error "We don't support that version yet..."
#endif

#ifdef _WIN32 // Includes both 32 bit and 64 bit
        #ifdef _WIN64
            printf("Windows 64 bit\n");
        #else
            printf("Windows 32 bit\n");
        #endif
#elif __linux__
    printf("Linux\n");
#elif __unix__
    printf("Other unix OS\n");
#else
    printf("Unidentified OS\n");
#endif

#if __APPLE__
        #include "TargetConditionals.h"
        #if TARGET_OS_IPHONE && TARGET_IPHONE_SIMULATOR
            printf("iPhone stimulator\n");
        #elif TARGET_OS_IPHONE
            printf("iPhone\n");
        #elif TARGET_OS_MAC
            printf("MacOS\n");
        #else
            printf("Other Apple OS\n");
        #endif
    #else
        printf("Not an Apple OS\n");
    #endif

#if __linux__
        // linux optimized code (will fail in other platforms)
    #else
        // general code for all platforms
    #endif
    // general code

using namespace std;

#endif // OS_H

I cannot get this straight. My issue is somewhere. It's my first time with os flags. I am not used to this.