While AirTags use Bluetooth (and Ultra Wideband UWB), the Bluetooth transmission isn’t in iBeacon format. The advertising is more complex and involves a rotating key scheme for enhanced security.
The Find My mechanism has been reverse engineered (pdf) by the Secure Mobile Networking Lab of Technical University of Darmstadt.
The researchers have an open source framework called OpenHaystack on GitHub that demonstrates how to create your own tracking tags by installing Bluetooth firmware on Linux, ESP32 or Nordic nRF51.
If you are a manufacturer wanting to use the Find My mechanism, you shouldn’t rely on reverse engineering that can change and should instead join Apple’s Find My network accessory program.