Possible solution to turn iPhone into wireless mic

https://itunes.apple.com/us/app/recostudio-multicam/id576184759?mt=8 This app allows users to control multiple devices by switching among them. This could likely be the solution of using one device as a wireless audio input. It is also known as Switcher Studio https://itunes.apple.com/nz/app/switcher-studio/id908386221?mt=8

https://www.imore.com/how-get-good-audio-while-shooting-video-your-iphone This approach is a more traditional approach, using one device to record video and the other to capture audio. Later import them into editing software and combine them together.

https://www.provideocoalition.com/moviepro-for-ios-how-to-set-180-degree-shutter-framerate-and-more/ (MoviePro + Airpods) According to this source that MoviePro uniquely supports Bluetooth microphones like Apple AirPods.

https://airlinc.io/ This app enables users to turn your iPhone into a wireless mic, but not sure whether the audio can be synced with video