Eagle-eyed readers may already know this, but despite the build.prop text saying otherwise, the picture above does not depict an actual Sony Xperia X, Sony's upcoming flagship handset that will hopefully propel them back to smartphone relevance. Instead, it shows an Xperia Z5 unit belonging to XDA user Jozinek, who found the full system dump for the Xperia X and decided to give it a whirl on his Z5. While it took some doing and is far from bug free, as you can see in the video below, it runs fairly well and just about everything works. As far as anybody trying to read what model the phone is, such as the Play Store, are concerned, the phone shown above is indeed a Sony Xperia X.
The system dump was pulled from the single-sim F5121 model of the Xperia X, codenamed Suzu, as seen above. New animations, a new lock screen and a number of other changes are on offer and work, for the most part, just fine on the Z5. The version of Android on offer here is version 6.0.1 Marshmallow, the most recent version of Android available outside of the Android N Developer Preview. Not every phone can run the port straight in this manner, however; most phones, even other Xperia devices, can only run certain apps and they must be ported over by modifying the code. The apps in question can be deodexed, which strips them of the specific optimization code that makes them run as well as they do on the Xperia X, in order to have their code modified to make them compatible with other devices.
The system dump was found by Jozinek on the Xperia Firmware website's tracker, at which point he took to XDA to share it with the community. At this point, it's mostly only useful to developers and very brave and knowledgeable Xperia Z5 owners, at this point, since most bits of it are still in the process of being optimized for porting to other systems. If you'd like to poke through the files or check out the full system dump for yourself, of course, it's available on both Google Drive and Deposit Files through the source link.