Android 7.x (Nougat) image was created using a stock Android image from Google. Several popular applications (apps) were populated with user data utilizing the capabilities of each individual app. The stock Android apps were also populated with user data.
Android 8 image is a LG GSM H790 Nexus 5X cell phone. It is distributed as a compressed tar file.
Android 9 image is a dump of a Google Pixel 3 phone running the Android 9 operating system.