This variable holds one of various constant GameMaker Studio 2 has to tell you which operating system the game has been created for. Note that this is not necessarily the same as the OS of the device running it, since - for example - your game could be running on an Amazon Fire OS, but will have been built for the Android platform (in which case os_type will be os_android).
The following constants can be returned:
Constant Description os_windows Windows OS os_uwp Windows 10 Universal Windows Platform os_linux Linux os_macosx macOS X os_ios iOS (iPhone, iPad, iPod Touch) os_tvos Apple tvOS os_android Android os_ps4 Sony PlayStation 4 os_xboxone Microsoft XBox One os_unknown unknown OS
case os_windows: global.Config = 0; break;
case os_android: global.Config = 1; break;
case os_linux: global.Config = 2; break;
case os_macosx: global.Config = 3; break;
case os_ios: global.Config = 4; break;
case os_winphone: global.Config = 5; break;
The above code checks the OS running the game and sets a global variable accordingly.