As seen in the video below, Lee created a 3D printed iPhone case and outfitted it with a built-in Lemaker HiKey board, a battery pack, and other hardware so it could support a version of Android. The case plugs into an iPhone's Lightning port, turning the iPhone into a display and emulating touch events on Android. While the iPhone is able to display the Android operating system, the Android OS itself is powered by the hardware in the case.
Lee outlines the case's creation process in a detailed post on Medium, explaining that he figured out how to clone the Android Open Source Project to make a customized version of Android Marshmallow, which is what is displayed on the iPhone.
Over several design iterations and experiments with 3D printing, Lee was able to shrink the case containing the parts down to a reasonable size and perfect the connection between the case and the iPhone. The result is an relatively thick iPhone case that lets the iPhone display and control a full version of Android.
Like Windows 95 running on the Apple Watch, Lee's Android case is conceptual and not practical for real world use, but it's an interesting take on getting Android to work with an iPhone.