iphone 7 running ios 15.7.9

There's the online iPhone User Guide. Many of the individual pages have pop-up menus where you can select a version of iOS. On one page, the choices went back to iOS 16. On another, they went back to iOS 12.

iPhone User Guide - Apple Support

A few days ago, Apple released a security patch to iOS 15, so there's now an iOS 15.8.

About iOS 15 Updates - Apple Support