AirTag can’t be used to recover stolen item if thief has an iPhone?
AirTags are not marketed by Apple as an anti-theft solution. It is for lost items only (i.e. when you can't remember where you left your car keys in your home).
If an AirTag is stolen, the thieve can detect the tag (given that they have an iPhone running a sufficiently new version of iOS) and disable it. Indeed even if they haven't, the AirTag will after a time emit a sound when moved, alerting the thief to its presence. However, you will still be able to see its last known location in Find My.