There are three methods on iPhone: 1. NFC 2. USB - this is actually how it works with the lightning connector on iPhone in Safari - the Yubico model with a ligh

There are three methods on iPhone: 1. NFC 2. USB - this is actually how it works with the lightning connector on iPhone in Safari - the Yubico model with a lightning port is working as a lightning to USB adapter in this case. 3. MFI (Made for iPhone) - some third party browsers can use the external accessory framework to talk to the Yubico model over the lightning port. Safari does not support this, but luckily can just use these keys as USB keys. I believe there is a hardware limitation that the MFI protocol only works over the lightning port and not the USB-C port, so they work for third party browsers on iPhone and not iPad Pro or the new iPad Air.