Thanks Rich, that link was helpful. Here is what I found after some trial and error. Getting the audio from my MOTU interface connected to iMovie was not an issue but not the video from my iPhone, even after getting QuickTime to recognize the iPhone as a camera input. iMovie recognizes the iPhone but only allows access to photos, not the camera, at least on my late 2013 iMac.
But I am able to record audio from the MOTU and video from the iPhone using QuickTime on the iMac, but the resulting file ends up on the iPhone. I then transfer the file to my iMac and import to iMove. A bit convoluted but it is working.
BR,
David