external microphone

I'd say there likely isn't a hardware solution, unless you find some kind of Bluetooth receiver that splits inputs and outputs.

The nature of iOS devices is that whenever something is plugged into the 3.5mm jack on older iPhones the audio output and input will both be switched to that one port. I assume the same goes for audio interfaces plugged in via Lightning on newer models.

As far as I know there isn't a hardware solution for this, but with a bit of Googling you might happen to stumble across something that meets your needs.

Best of luck!

Unfortunately this is not supported natively in iOS.