iPhone Lightning port can be used as a "Line In" connection to route audio from either a mic or another source, such as your amplifier [with appropriate jack of course or RCA to mini jack adapter].
So, if like me you wanted to digitise your vast collection of vinyl records with all the 'character' that vinyl gives you simply need the correct adapter.
My Apple Lighting - mini jack would NOT do the trick. I had to use a third party product to force the iPhone [in my case both 7 and Pro 14] to "see" my connection and treat it as an audio in port.
I used the dongle that came with my Røde mic. It is made by BOYA and is the BY-K8 dongle. It works a treat.
Note: there is a new TRRS which has replaced the one I am using the TRS mini jack.
I hope this helps all you guys who want a simple "Line-In" solution from any audio source to your iPhone. It is a superb way to digitise your audio collection and get it on your digital library once and for all!