I'd bet offloaded and scanned at airport, IF they don't go to a sorting facility at that point.
I think it goes like this, if my years in the parts business is any indication:
1: origin departure. In this case China.
2: arrival at airport near sorting facility. In this case Memphis.
3

hones are on pallets to the tune of 500 or so units. I'd imagine that each pallet is a specific model, ie, iPhone 6 Plus 128gb Space Grey for Verizon, next pallet Iphone 6 64gb Silver AT&T, etc. It's possible that one single plane is full of different models for a single carrier, but I doubt it.
4: these are sorted and processed in the sorting facility, which is very likely off site from the airport. This is where they are assigned to customers, packaged with shipping labels, then loaded onto the FedEx truck. This is also the point where our info is updated in Iconic, shipping emails are processed to be sent, and the package is headed to your doorstep.
5: phones flies it's little behind to your local sorting facility overnight where it's out for shipment the next morning.
Granted, this is all taken from my experience ordering and shipping auto parts, but the whole idea is identical. But Apple ships more phones in one week than I've shipped my entire career, so imagine the real complexity of my watered down version.