Just worked this out, this is for iPhone 6, however iPhone 6 plus should be similar.
The NFC antenna is the top bezel itself, the top bezel has 2.4G(wifi and Bluetooth), cellular diversity, GPS and NFC. The main feed point for GPS, cellular and 2.4G is the screw boss under the plastic add-on antenna, with a "grounding bracket" attaching it to the bezel. The plastic antenna serves as the 5G wifi antenna.
The NFC antenna feed point is the screw boss next to the camera and sleep/wake button, with a flex cable under the camera bracket serving as signal conductor.
There is an additional feed point in the middle of the bracket, it does not connect to any wireless services, however it connects to a dynamic switch to tune the top bezel dynamically to serve different bands.
The F-shaped flex cable on the back of the board is JUST a bridging cable, not an antenna in any way.