What is the radio frequency of Airdrop transmission?

The protocol is called AWDL, trying googling that. Here's a starter

https://stackoverflow.com/questions/19587701/what-is-awdl-apple-wireless-direct-link-and-how-does-it-work

AirDrop uses Bluetooth and WiFi to broadcast beacons and identify and negotiate a peer to peer mesh link. Since the WiFi radios are capable of multiple transmissions, they will hop and change channels based on the conditions at the time of the construction of the peer to peer mesh.

Since there are far more 5 GHz channels available than 2.4 GHz (and they are faster and they suffer less from overlap and suffer less in interference) I would presume the chances are highly stacked to prefer a free 5 GHz channel, but it’s not guaranteed this happens when you measure the actual traffic.

You will need sophisticated hardware to capture this traffic by listening and as it’s encrypted, even once you capture it, the contents are likely going to be difficult to decrypt.