How any chat app is made?

Answer 1

there are 2 main protocol in chat/messaging. IRC and XMPP, IRC is kinda rare now but there are some community that actively using IRC. XMPP used by Whatsapp, i read it somewhere in the net not sure 100% though.

but because the rise of websocket, and imo its much easier to setup maybe websocket is a good way to go.

Answer 2

Through the use of websockets (in any coding language) for real time communication.