I was wondering what is the best way of going about executing code that is dependent on the current run time platform?

The one way off the top of my head I can think of is something like this:

 
int main()
{
    //#define IOS
    #define WINDOWS
    //#define ANDROID
 
    #ifdef IOS
           DoIOSCode();
    #elseifdef WINDOWS
           DoWindowsCode();
    #elseifdef ANDROID
           DoAndroidCode();
    
    return 1;
}
 

But I don't think #defines are globally recognized right?

You dont need to define these yourself. Most compilers have premade defines, which you can use to detect the OS, compiler and some language features.

Projects often create a header that (among other things) checks the built in macros in a series of ifdefs and then defines own macros for the platforms like MYPROJECT_WIN32, MYPROJECT_LINUX, ect. to make it consistent because the built in platform or compiler macros often use different naming styles.

And defines are recognized from the point where they are defined till they are undefined. They happen in preprocessor and are just pure text operation, they don't respect scopes, namespaces or anything. Include is a preprocessor feature too and is basically like a paste of entire file in place of the #include <> line, so a define in included file affects everything below it and in the files that included it. You can play with 'cpp' or 'gcc -E' on Linux (or Windows or any system where you can get these tools) to see the result of just preprocessing being ran on a file.

You dont need to define these yourself. Most compilers have premade defines, which you can use to detect the OS, compiler and some language features.