Find My-tracked devices emit a Bluetooth chirp. If any Internet-enabled iOS devices hears it, it forwards to iCloud/Apple backend.

I can lose a WiFi-only iPad, and it can be fully off any WiFi connection, and still get a ping on its location if its Bluetooth chirps get relayed by a stranger’s iPhone.

This exists now, for i.e. the offline iPad example. This larger launch extends the Find My network to a new class of Internet-less, long battery life tag devices.