Apple's first foldable iPhone runs a meaningfully different version of iOS.
According to a report detailing the iPhone Duo's interface, Apple has rewritten parts of iOS to account for the device's foldable design. The changes reach the camera system, which has to adapt to a hinge and dual screens, and multitasking, where a bigger unfolded display changes how apps can share screen space. Other core behaviors get adjusted too, beyond what a simple scaled-up interface would require.
That's the harder problem foldables have always faced. Samsung has spent years teaching Android to handle a crease down the middle of the Galaxy Z Fold's screen, and app support is still inconsistent. Apple entering this late means it had years to watch those growing pains before writing a line of code.
Software, not the hinge, will decide whether the iPhone Duo feels like a real computer or a phone wearing a costume.