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 click Download All Purchased Tones to get the ringtones synced to the new device.

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