WebSocket Server in Zoom Clone Coding

Hi everyone.

I'd like to ask the reason why Nico used Web Socket server as a 'signaling server' in Zoom Clone Coding lecture(WebRTC Lecture).

Should it be the duel-flex server?
It can not be a normal HTTP server or normal Express Server?
or Does anybody have any clues about that Web Socket?

Please feel free to answer my wonder.
Thanks.