Are they describing a TURN server, or another solution?
TURN stands for Traversal Using Relay NAT, so it seems they have something beyond TURN. Nevertheless, the architecture is probably similar.
They also talk a lot about WebSockets end using Elixir, at least for their Signaling server.
WebRTC doesn’t standardize technologies to be used in the signalling layer, so the use of WebSockets is absolutely valid.
What is the involvement of Elixir in their (or another WebRTC) application?
Don’t really understand this question. It’s possible to implement entire WebRTC stack in Elixir, we aim to do that at Membrane.
Could you also transfer video & audio through Websockets?
Video or audio is just a stream of data, so you can transfer (send and receive) them anyhow. The problem is that live streaming is way more complex than just sending and receiving data. You can see this talk by @mspanc to have some overview. Proper tooling can handle significant part of that complexity.
