I haven't bought into Apple stuff too much, just an iPad and an Air. No iPhone, I have Android instead and my desktop is Linux. Question: Is the phone a mere a

I haven't bought into Apple stuff too much, just an iPad and an Air. No iPhone, I have Android instead and my desktop is Linux. Question: Is the phone a mere accessory to the iPhone, or can it stand alone or with any phone (inc' Android and Windows Phone)?