Its amazing how many people just come her to post and dont even go into Apple's own website to check. I have seen some post asking questions which can be solved by going to
https://www.apple.com/airtag/
1 - How does it work? 30m only etc.... It reports its location to ALL IPHONES that are within 30m.
"Get by with a little help from hundreds of millions of friends.
When youʼve left something far behind, like at the beach or the gym, the Find My network — hundreds of millions of iPhone, iPad, and Mac devices around the world — helps track down your AirTag. And itʼs designed to protect your privacy every step of the way."
2- Does it work in public transportation? Will I get ping if I am riding with somebody etc?
"AirTag is designed to discourage unwanted tracking. If someone else’s AirTag finds its way into your stuff, your iPhone will notice it’s traveling with you and send you an alert. After a while, if you still haven’t found it, the AirTag will start playing a sound to let you know it’s there.
Of course, if you happen to be with a friend who has an AirTag, or on a train with a whole bunch of people with AirTag, don’t worry. These alerts are triggered only when an AirTag is separated from its owner."
3- how long does it have to travel with you before it notifies you?
if you look at the picture in Apple.com/airtag/ you can see it is 9:41am and it is showing a notification that an Airtag is detected near you. It says it was first seen at 8:50am, arguably this means that within the hour of a tag traveling with you and WITHOUT its owner.... it will notify you.