My recording setup for using an external mic plugged into an iPhone for video:
1) Zoom H4n Pro or summat (https://zoomcorp.com/en/us/handheld-recorders/) (what I use for my audio these days)
2) Long cable headphone jack male (plugged into Zoom H4n headphone jack) to female; for length so I don’t have to put the camera near the recorder if I don’t want to; I usually put the mic up high and behind me or off to my drone side so I don’t end up with chanter heavy recordings.
3) a Rode TRS male (plugged into the cable above) to TRRS male adapter, SC7 I think, the red one (http://www.rode.com/accessories/sc7) – this is required because Apple is TRRS (tip ring ring sleeve), not the usual TRS (tip ring sleeve = normal stereo)
4) Apple TRRS (plugged into the Rode) to lightning adapter into my iPhone using just the video mode in the camera app (https://www.apple.com/shop/product/MMX62AM/A/lightning-to-35-mm-headphone-jack-adapter)
Looks like the H1n at $120 would be a good option for excellent audio without the H4n price.