Smartphone audio SUCKS!! However the super compact RØDE Wireless GO fixes that super easily and sounds awesome. Headphones ON please. Check out the Wireless GO's Built-In mic the sound straight out of my iPhone X. You'll need the SC7 cable (TRS to TRRS) adapter to use the Wireless GO with your iPhone. It's not expensive and totally worth it.
My other RØDE Wireless GO videos
3 different RØDE Wireless GO setups tested simultaneously plus STEREO Wireless GOs outdoors in Tokyo: https://youtu.be/uh-BGPDJHmI