Long time lurker, first time poster. I finally cracked the code for getting op1 audio to record directly to the iphones native camera app when recording video!
Video I recorded using my method: video!
DIRECTIONS:
aux cable → cheapo $5 audio interface thingy → usb Iphone camera connector kit → iphone 11 pro
links to products at the bottom!
the only way I got the monitoring to work while using video was with an old beatmaking app called “Beatmaker 2”!
I remembered that it was always still monitoring while i went out of the app a few years ago and its literally the only way i’ve gotten this setup to work perfectly. open the app, new audio track and turn on monitoring. it’ll still monitor while youre texting, scrolling reddit and recording video on the native app!
dug the internet for months and never found any way to do this. I even messaged TE for help and they didn’t know a work around either. I really hope this helps someone desperate like i was. don’t buy that $100 line 6 audio interface or whatever it doesnt work with ios 13
LINKS TO WHAT I USED:
(remove stars before pasting links)
USB Sound Card Adapter: http***s://www.ebay.com/itm/373129262567
$10 and under / not the exact one I use but literally any of them should work. make sure it has headphone output as well.
Apple Lightening to USB Camera Connection Adapter: http***s://www.bestbuy.com/site/apple-lightning-to-usb-camera-adapter-white/7006933.p?skuId=7006933
$33 / regular usb, the fancier one with usbc hisses when recording
Beatmaker2: http***s://apps.apple.com/us/app/beatmaker-2/id417020234
$??
everything + a set of decent mixing headphones is still cheaper than a lot of audio interfaces i’ve seen recommended to try this out! if anyone could confirm if it works on iPad as well I would really appreciate it. 
happy instagramming!
denyel