To set a custom ringtone on an iPhone, first we need to purchase it through the iTunes Store. The purchased ringtones will appear under Settings > Sounds & Haptics > Ringtone. I don't see those in the Tone Store listed anywhere. When moving to a new phone, sign in to the Apple account and tap Download All Purchased Tones to get the ringtones synced to the new device. There are manual ways to set ringtones, which are not discussed here.

I use Fringe - Main Title from the Fox TV Series as my ringtone since I started using an iPhone, which is from 2017 onwards.