I have not worked on one of these yet. But Apple loves little bits like these. It looks like an antenna of some kind. Based on the way its shape I would wager it goes into the side of the housing somewhere, like a button flex. My first through would be the NFC antenna which is usually in the sidewall of the housing near the rear camera. But I haven’t been able to pin down the location for sure.
Here’s a pretty in depth teardown for the 14 Plus on a top down view. You might be able to find the placement based on this.
https://youtu.be/QlkC6_AiinU?si=9s53xDZBbD-j2KWf