Many external microphones that are used with DSLR or mirrorless cameras connect with a TRS cable. One end goes into the microphone and the other end goes into the camera’s microphone jack.
However, smartphones do not have a separate microphone jack. The external mic usually connects to the headphone jack. Because of this, if you plug the same TRS cable you use with your camera into a phone, you won’t get audio.
Instead, you will need a TRS to TRRS cable. The TRS side — which has two rings on the plug — plugs into the microphone jack like normal. The TRRS side features a third ring on the plug for the microphone channel. When you plug this into your phone, you will see audio showing up.
Using a compact external mic for smartphones can provide superior audio recording quality, so I recommend choosing this option over AirPods or the phone’s built-in microphone. Be aware that you may have to unplug the TRRS cable to hear audio playback from your phone since the phone assumes headphones are plugged into the headphone jack.