Original usb-c to jack 3.5 does not work on FP4

[yvmuell]

I guess that applies to your use case as well. This USB-c to jack cable cant work this way it does not include any DAC and is meant to work the other way around on the XL Buds

Question regarding Jack-to-usbC cables

If I understand you correctly, you are trying to plug the jack into Phone or Steam Deck and USBc into your headset. I would guess, that your research was kind of right and the cables you tried are the other way round: USBc to Jack and not Jack to USBc. For Jack to USBc you can e.g. use this cable: https://shop.fairphone.com/de/shop/fairbuds-xl-headphones-usb-c-mini-jack-3-5mm-cable-64?category=5