"This application requires iOS 15.0 or later"

Before you go ahead and buy a new iPhone because of the others who have replied to this thread, I would like to tell you that there is a workaround to this issue. The workaround is to purchase it on a newer device so it will be saved to the purchased section in your Apple ID, after which you can go ahead and install it on the iPhone with the older iOS version, such as iOS 12.5.7. It will allow you to install the latest compatible version instead.

Unfortunately, the iPhone 6 is not compatible with iOS 13 or higher.

That model can be updated to iOS 12.5.7, no further.

If you need to run apps that require iOS 15 or iOS 16, you will need a new phone.

Eventually some of these things will become outdated, though.

Only if the developer has not prevented earlier versions from being available.

Yes, that is so, but for now most apps work with this method, including YouTube, Google Chrome, etc.

While it may work for YouTube it is not required. Nor for Instagram.