List of connected devices to local wifi network

Here you have an iOS LAN scanner

https://github.com/mongizaidi/LAN-Scan

And you can take a look into bonjour too https://developer.apple.com/bonjour/index.html

Bonjour, also known as zero-configuration networking, enables automatic discovery of devices and services on a local network using industry standard IP protocols. Bonjour makes it easy to discover, publish, and resolve network services with a sophisticated, yet easy-to-use, programming interface that is accessible from Cocoa, Ruby, Python, and other languages.

One way to realize this is pinging all IP addresses in your WLAN. If you get response from an IP address, you find a neighbour. It's simple but slow, and may miss the device which refuse to respond to ping.