They could be, easily so, twrp backup (and clockworkmod recovery before that), you'd just create the image of the phone (all partitions, everything), dump it to

They could be, easily so, twrp backup (and clockworkmod recovery before that), you'd just create the image of the phone (all partitions, everything), dump it to wherever, and then restore it, it was a standard system for testing new android versions on the phone. The "secure enclaves" and other related stuff have made this harder in the recent years.