Where does AirDrop save files on macOS?
Look in /private/tmp.
I have mixed reliability with AirDrop – the sending machine is usually able to send all the bits, but the receiving machine doesn’t always put the file somewhere I can find it, say my Downloads folder.
Every so often I see the temporary folder where AirDropped files are saved before they’re copied to a more findable location – it’s /private/tmp. If I’ve received a file using AirDrop but it’s not appearing anywhere, look there.