How to fix update, and download from Apple store

So my brother has an iPhone 8 and it’s still stuck on iOS 12.5.7 (which I have no idea how) bout everytime he goes to download a game or anything on Apple Store it tells him he needs to update to iOS 16. Well whenever he gets to his general setting where the updates would normally be it says he is caught up to date with the iOS 12.5.7? I don’t understand how to fix it in order for him to have the correct update or download anything wise?