The headphone jack is inside USB-C connector. You can use USB-C-to-3.5mm passive (!) cable to connect your 3.5mm headphones, or you can buy 3.5mm headphones with USB-C audio connection.
There's no additional DAC involved, the analog audio is already on a USB-C pins.
I can't find anything about that in the article or the Fairphone 6 manual. Do you have a source for this?
My main problem with USB-C headphones on my Android phone is how unreliable they are. But if the USB-C port can just pass through an analog audio connection, that should work much better, I expect.
There's no additional DAC involved, the analog audio is already on a USB-C pins.