I use the Blackmagic video app. It has an indicator for microphone input levels. I tap in the microphone and it barely picks anything up, but tap on the phone and it is loud - indicating the internal microphones are the ones being used. When I plug in the microphone alone, the phone switches to it. Besides that, the microphone has a light on it to indicate it is being used. When plugged in alone the light comes on, but when plugged in with anything else also plugged in, the light does not come on.
In any case, the audio gets recorded, because nothing cuts off all microphone input - it just defaults to the phone's microphones if the external microphone is not detected. Apple has some unusual circuits in the Iphone and it requires the use of certain cables, etc. For example: using a small extender in the usb c plug will always disable use of the microphone. If a longer usb c cable, especially if made by Apple, will usually activate the microphone. Like I said, even the Apple staff doesn't seem to have a good understanding of how this works.