I want to use the Air Tag for tracking my car. What’s the range of an AirTag?

It's Bluetooth, so about 35 feet.

It's designed to be a beacon, it broadcasts it's information wich is picked up by nearby iPhones and communicated to a server so you can see it.

If no iPhones are nearby, you won't see it's location.

the air tag does not send directly a signal to your phone, but uses other ways, which are unlimited in principle (only it must be on earth somewhere having diverse networks in the vicinity)

Be aware that tracking someone else is illeagal