Will the iPhone prefer using ethernet via the lightning port, if it is plugged in, instead of using cellphone bandwidth?

I have never used an ethernet adapter with an iPhone, but I find it likely that the iPhone will use the wired ethernet connection instead of cellular data regardless of whether or not airplane mode is on, similarly to how WiFi is preferred to cellular when connected to a WiFi network.

Since using a wired Ethernet connection is undocumented, there is no guarantee at all that it will always work the same from one iOS release to the next. It may happen to work now, but Apple is not in any way obligated to make sure it always works or that it always works in exactly the same way.

I use a TP-Link AC750 portable wireless router in my RV. It is capable of reliably sharing a wired or wireless network connection to any wireless device. Cost around $35, which is probably less than all the needed gear to get an iPhone to directly connect to a wired network. My iPhone and other devices always see it as the same WiFi access point, no matter where or how I’m using it to get internet access.