Is it transferred by WiFi or Bluetooth when you use Airdrop? I get mixed info.

Both are used, but mainly Wifi for the transfer & BT for discovery...

How AirDrop Works

AirDrop uses an interesting combination of technologies to transfer files securely. It uses Bluetooth to find devices that you can send to, and the device you send from creates a secure peer-to-peer Wi-Fi network connection with the receiving device and transfers the file(s). This transfer does not use the Internet, nor a local Wi-Fi network; you don’t need to be on a Wi-Fi network to use AirDrop. This makes it practical anywhere, such as in the field, where you may want to transfer files from, say, an iPhone to a MacBook Pro.

https://www.intego.com/mac-security-blog/use-airdrop-to-securely-transfer-files-across-your-devices/