Hi Zoe,
Sounds like a somewhat common mistake we see on iPhones where if you forget to take out the two Pentalobe screws next to the charging port but then try to remove the screen anyway, you end up separating the frame that's supposed to be mounted on the display from the display itself, leaving the screen frame attached to the phone's chassis.
Here's another case where that same thing happened to them.
SOLVED: Why does the inside of my iPhone x look different than everyone elses? - iPhone X - iFixit
iPhone XS replacement screen doesn't fit - iPhone XS - iFixit
All you need to do is first make sure you remove those Pentalobe screws then go back around the edge of the chassis and repeat the screen removal procedure on the frame this time. If you're not replacing the screen you'll need to reglue the frame back onto the display; otherwise just salvage the sensor array from the separated frame and toss the old frame and screen.
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