Adapter from IPhone 6 charger to USB?

So my new headphones are IPhone 7 style, aka they have the IPhone charger as their plug, and my computer is older, so I can't plug it in. I'm trying to find an adapter from an IPhone charger to a headphone port, but can't find that anywhere - it doesn't seem to exist?

Lightning is the name of the iPhone charger port for the record.

As best as I could tell your situation seems to be the reverse of what most encounter. I have the old headphone jack headphones (using from my iPhone 6) & using the adapter from my iPhone 7 to use on my iPhone 7.

There's no such thing.

The Apple "Lightning" port is proprietary to Apple iphones and is actually a digital expansion bus system.
No one else makes anything with a lightning socket & any pluggable items or leads must be licensed by Apple.

Your lightning-plug headphones actually contain the lightning bus equivalent of a PC sound card in the plug, to run the headphones…
It's an extremely complex package:
https://d3nevzfk7ii3be.cloudfront.net/igi/fTLqEA54MvcKMNdy.medium