Live streaming technologies with Elixir / Phoenix

I understand some of the advantages of WebRTC, but I’m still somewhat confused about how to use it in a centralized server architecture, like the one described by Discord.

Routing audio/video through media servers offers other advantages as well, such as moderation.

Are they describing a TURN server, or another solution?
They also talk a lot about WebSockets end using Elixir, at least for their Signaling server.
What is the involvement of Elixir in their (or another WebRTC) application?
Could you also transfer video & audio through Websockets?