In addition to the other replies, it is important to understand that an Apple AirTag is NOT an anti-theft device for a number of reasons.
One is what has already be stated ... and that is the tag relies on Bluetooth and the Apple "Find My Network" to relay it location to the Apple servers over the Internet. That is where you would find updated location information with the Find My app on your iPhone.
Two, depending on where you placed it in the car. The tag's Bluetooth signal may not be readably available to a nearby iPhone. In addition, the tag only broadcasts this Bluetooth signal once every 3-5 mins to conserve it battery.
Third, when the AirTag is separated from it paired iPhone, after a certain amount of time, it will start beeping. This could alert a potential car thief of its presence.
Finally, if that thief also happens to have an iPhone, that phone receive a notification that there is an AirTag "following" them.
As you can see, this make for a poor anti-theft device. If your goal is to track your car, should it get stolen, then you need to consider getting a dedicate anti-theft tracking device. One of those would be something from LoJack which is based on GPS, and not Bluetooth.