Why would port 19421 be open on an iPhone? Following up on the Zoom vulnerability, I decided to run nmap and scan my local network for devices with port 19421 open. One of the devices turned out to be my iPhone!  What service would use that port on an iPhone, and should I be concerned about it?
It's not a standard service. I would suggest rebooting the iPhone and checking again. Then you can start one app after another until you get to one that opens that port. Possibly the port number is dynamically allocated, so that you wouldn't fall upon the same port number again.