Say you want to do Facebook Live but have good audio, and happen to have a Tascam 70D audio recorder. It’s super easy to connect it to your iPhone. This should also work with other Tascam recorders that have 1/8″ Line Out/Camera Out jacks, though the setting might be named differently.
Buy one of these adapters, plug into phone
Connect the Tascam’s Line Out to the Microphone side of that adapter using a standard 1/8″ cable (this one is coiled and nice)
Be aware that, even though the TRRS adapter has a Headphone connector, I couldn’t get audio out while streaming to Facebook Live. But if you set it up as I did above, you can monitor on the Tascam and be (reasonably) confident it’s the same on FB. At least until Facebook randomly changes something on us.