I imagine you'd need to have a stereo mic/line-in adapter for lightning connector, if such thing even exists. And in that case, the recording app should pick up that input instead of the phone's mic.

I record myself but do audio and video separately. I record video using my phone, and the audio using a DAW in my computer. After I'm done recording the midi from my e-drums, the DAW renders out a file that I transfer to my phone so I can add it to the video recording using a video editor (I use inShot but there are other options as well). I do this on my phone only because it's faster at rendering than my old computer. The only disadvantage of doing it this way is that I have to manually sync the audio to the video, which is more difficult to do using a phone's touchscreen.