There are adapters with 4 connections that will feed a mini-jack/lightning adapter. You can get them with screw terminals, or you can get them with particular connectors.
On YT there’s a guy who shows how he connects his mixer board to an iphone using the lightningheadphone thing with a y cable.
Doesn’t work (for me, tried it).
My puzzle was how to get sound into an iphone (video) without the hassle of cck+audiointerface+power etc. As ‘light’ as possible. Not so easy