Unable to use WhatsApp on iPhone 6

I am unable to use whats app in iPhone 6

And that would be because the developers at WhatsApp decided that they would require at least an iPhone 7 or higher model to run their app.

This was WhatsApp's decision. Apple had nothing to do with this.

That app, and many other popular apps, do not support the iPhone 6 - a phone introduced about eleven and a half years ago that cannot update to iOS 13 or higher.

Yep. WhatsApp requires iOS 15 or newer. That phone is limited to 12 I believe.