When the original Apple Watch was released back in April 2015, one of the features that appeared to get lots of attention at the time was the ability for the Watch to unlock doors (https://9to5mac.com/2015/04/24/apple-watch-hotel-room-key/ is a good example of this). From a technical point-of-view, how is the Watch able to unlock doors in this way – is it purely based on the NFC chip inside the Watch, or does it rely on a companion iOS app running in the background on the paired iPhone?
From a technical point-of-view, how is the Apple Watch able to unlock doors
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Best Answer
The example you reference is actually not implemented using NFC at all.
Instead the lock in the door is Bluetooth-enabled, and the app on the Apple Watch thus uses Bluetooth to communicate with the lock for unlocking. Today the SPG branded app has changed into a Marriot-Bonvoy branded app, but still works on the same principle. If you look at the FAQ for the app, you'll notice that it instructs the user to ensure that Bluetooth is On.
The process does seem to rely on a companion app on the iPhone for initial setup at least. From screenshots it seems the Apple Watch app can show the error message "Phone unreachable" during unlocking, which seem to indicate that the phone needs to be present.