Suppose I launched my app and kept it is in background by pressing home button and now user is started using their other apps (like WhatsApp, Instagram, Paytm etc any apps) .
My requirement is that I need to detect that user is using their phone to use any other app or not.
In short, your requirements are malicious since this would be a huge security breach.
You cannot access other applications, you cannot even check they are installed unless they support it themselves.
The only possibility is having a jailbroken iPhone, that is, an iPhone with security measures turned off and direct (hacked) access to system functionality.