Apple Air Tags are great. Cheap, they work and work well. One must keep in mind though, for many, including air tags, these are not GPS transmitters. Good news is they cost almost nothing and there are no fee's of any kind.
The way an AirTag works, and works well since 50% of the USA population has an iPhone is the AirTag will transmit its location if lost or stolen when it makes contact with another iPhone. It will relay the location using a strangers iPhone.
We have many, one on our dog too!
We also gave them as presents, a real world case was our daughter last year, if that long. She put one in her suitcase. Well, got off the plane one day and her suitcase was gone. The AirTag showed the suitcase on the interstate outside the airport. *LOL*
She said she didnt have much value in the case and really didnt jump on it until it became apparent that someone wasnt going to return it. We actually googled its last known location at a house, but again, our daugher didnt do anything for days and finally had the cops go over and ask the people, they denied taking it and said if it was anyone it would be their neighbors across the street because they deal drugs. Nothing much the cops could do after that according to them. Tracking devices can be thrown away and they weren't going to search a house.
Anyway, the point is, they work, she just let a lot of time go by before acting on it. She could have went over there right away.
The answer to another question in here is what would you do if you found them? Well, for one thing, if you had one on your dogs collar (maybe even cat), you would be happy that you did if they got lost!