victim's BLE address == target device they previously scanned and set up an Airtag for ?
Expensive computed public key first 46 bits == Victim's BLE address
The Apple FindMy system doesn't (or didn't) validate that the public key being broadcast had ever been manufactured or registered. So anyone with an iCloud account could query the Apple FindMy hashtable for the last observed encrypted payload, which contains the observed location generated by the nearby phone.
If you have root on the victim's device, you don't need the expensive computation step. You just take a public/private keypair of your choice and reprogram the victim's Bluetooth hardware to broadcast that instead.
What is novel in this attack is that you can use non-root access. You observe the victim's fixed Bluetooth address, and then craft a FindMy beacon that happens to match. Since the FindMy beacon is basically just a public key that the receiver uses to encrypt location data, crafting the beacon is just finding a public/private key pair that matches the victim's Bluetooth address. Since broadcasting a beacon requires less rights (less than root), this is much more broadly exploitable (excluding the expensive precomputation step).