The “cat /dev/brain” blog:

iOS 15.0 introduces a new feature: an iPhone can be located with Find My even while the iPhone is turned “off”. How does it work? Is it a security concern?

This is a bit of a techie rabbit hole, but I found it fascinating.

At its core is a discussion of the AOP, or Always On Processor:

All chips and various embedded devices Apple manufactures run a real-time operating system, called RTKitOS. The AOP on the iPhone is no exception. However, the AOP has a special role. It connects to almost every other chip in the iPhone. For some chips, it only does basic tasks like power management, and for other chips, it acts as a transparent proxy that wakes up iOS when needed.

This way, a processor that is always on actually saves energy. iOS can go to sleep while the AOP waits for hardware events. A simple example is the motion sensor. Without touching any button on the iPhone, the display wakes up.

If this sort of technical arcana is of interest, follow the headline link and dig in. Skip the stuff you don’t understand. After each technical deep dive, the author returns to the surface before discussing the next bit.