how to run whatsup and phone pay in iPhone 6s with ios 15.8.4 vers

WhatsApp can be used on your iPhone as it requires a minimum of iOS 15.1.

You likely cannot run PhonePe due to the rules the NCPI (National Payments Corporation of India) have put in place. UPI fraud and scams are very prevalent in India, so they asked for a feature to combat it. For iPhones it was introduced in iOS 17, so important that they were willing to abandon older iPhones support. Information is from a very knowledgeable user named Servant of Cats.

Meaning you’ll need to purchase a newer iPhone that can run a newer iOS.

The latest version of WhatsApp (25.16.81), released 4 days ago, is supported on iOS 15.1 and above, so you should not have any issues with iOS 15.8.4. I'm not sure what you mean by phone pay, please explain.