I am not a Boeing engineer, but I do have some experience in avionics design.

The flight critical systems would be isolated from in-flight entertainment system (IFE). The IFE probably has a listen-only tap to flight metric info on an ARINC databus from the flight computer. I did a quick search and looks like 787 uses ARINC 667, a fiber optic interconnect.

One way connection of some kind? Its not an air gap but may functionally work the same way.

Optoisolators are a prolific and an extremely cheap electrical component that performs the "air-gap" functionality.

Electrical safety airgapping and information security airgapping are different, and entirely unrelated. If data can pass from one side to the other it's not airgapped (from an infosec perspective).

Fiber needs transmitter and receiver transducers. If each side has only one of those, data can literally flow only in one direction. So it's still infosec-airgapped in the other.

No, data diodes are a different (but related) concept. An airgap implies no information transfer, either in or out. Use an airgap when information leakage must be stopped (as well as remote attacks), use a data diode when information can be released but you need to stop remote attacks.

Fiber optic connections aren't required, anything with separate transmit and receive lines can be turned into a data diode (as long as the protocols used permit it). RS232 null modem cables with the RX lines disconnected are a classic.

No, that's not airgapping.

I don't know enough about airplanes to know if it would be unencrypted but they radio all of that same data to the ground in some way, could the IFE system just passively listen to that broadcast?

GPS + external sensors for pressure, wheels, …