While the video is in Japanese, in the description, HARUKI explains how the tracker emits sound.
It seems that the voice coil motor placed in the central magnetic field vibrates due to changes in the current, and the housing on the coil side acts as a diaphragm.
AirTag contains a user replaceable CR2032 battery that offers up to a year of battery life.
Take a look at the video below and please download the iClarified app or follow iClarified on Twitter, Facebook, YouTube, and RSS to be notified when an English version is available.
You can purchase AirTag from here. It costs $29 each or $99 for four.