I've been out of the loop on foldable phones, and this is question is not related to specifically the iPhone Duo. How does it work that it doesn't show a crease in the middle? What's the breakthrough technology there? Or is there a visible crease?
For foldable phones, this might be a good pick to get caught up (5 months old, but minimal crease): https://www.youtube.com/watch?v=eFUB_jL_XcM