I'm not sure who added that article to the Media section, as it is only talking about SignalR usage for broadcasting messages and text data. It does not appear to be related to video or audio broadcasting. I'll see if I can locate the article owner still and have it removed from that section of the docs.
So, to answer your question. (a) Ignore that article on instant broadcasting, as it seems to be unrelated to your ask. If you are interested in realtime communications like Teams that uses WebRTC - look at the Azure Communications Services SDKs and documentation.
(b) AMS is not related to the above at all. Media Services provides live streaming for events and large scale broadcasts. You can find more information on AMS at http://media.azure