How? Airtags only work because Apple uses any iPhone with BLE support as a detector, whether or not an app is installed. An off-brand BLE beacon would require an app to work at all, and I highly doubt Apple would make it easy for a third party app to run all the time in the background.
According to https://developer.apple.com/find-my/ manufacturers of third party hardware CAN leverage the iPhone BLE network to connect to the FindMy network.