Well, that is why I use the word “hardly”.
HA can stream that single source, but none else.
It can not receive streams and it can not mix streams.
You need MPD or similar for that and then HA is only used to control it.
I use a setup with MPD to play my music collection and different radio streams.
MPD can mix it and receive streams and I then output to a Icecast2 server for devices to receive that mixed stream and to a shoutcast server for sync playing on ShoutCast client connected speakers and devices.
HA is here not creating or setting up streams, but only control them.
MPD is the one handling the actual input, mix and somewhat output. ShoutCast server and Icecast2 server just converts to their format and stream on.
The issue here is that MPD is not running on HA, at least not in a way that can handle this, because I think the music assistant is actually a MPD service.
That means you need another place to run the MPD client, like a separate server or maybe the hostOS on a supervised HA installation (This require a good knowledge of how the different programs work together and it is not recommended by the HA developers!).