Can I intall iOS 16 on iPhone 7 Plus?

No, you cannot. The iPhone 7 Plus is not compatible with iOS 16 and higher.

I would recommend against buying an old iPhone 8, 8 Plus, or X to run iOS 16. Already we are seeing some popular apps requiring a minimum of iOS 17, including the Netflix app, and all UPI payment apps in India. If your goal is to be able to run current apps, getting a phone that is compatible with iOS 17/18 makes more sense.

The iPhone 7 is not compatible with iOS 16 or higher.

If you want to run iOS 16, you will need an iPhone 8, iPhone 8 Plus or iPhone X.