Unable to install apps due to outdated software on iPhone

Sorry, but your “elderly” iPhone 7 can only run the iOS 15 you already have. (currently 15.8.5)

The inability to run apps is common on older phones as app developers drop support of older OSs with an ever-shrinking number of users.

Use caution if considering buying a used phone:

The All Too Common SAD Reality of Buying … - Apple Community

Newer apps require a newer iOS which in your case requires a newer phone.

FYI, the “oldest” phone capable of running the “newest” iOS is an iPhone 11

Yeah, the iPhone 7 fell off current iOS support ~four years ago.

Various third-party apps choose what iOS versions they support, and can support the three most recent versions.

Option: acquire an iPhone 11 or later, or an iPhone SE 2nd or later. That can run iOS 26 (2026).

The developers of the apps that you want to install on your phone have made the decision to no longer support older iOS versions.......because they want their apps to run on the newest iOS versions.

In effect, the developers of the apps have abandoned you and they are now forcing you to buy a new phone if you want to run their new apps.

Apple changed nothing. It is the app developers that have made the changes which will no longer support your iPhone.

Yes, the “7” went thru 6x iOS versions (10 to 15).

That’s fairly “usual.”