Well, for a long time I’ve been using Rogue Amoeba (a great name!) Airfoil. This plugs itself into the Mac sound system and lets you stream content to just about everything. From Chromecast, to anything else.
Apple introduce Airplay 2 which does the same thing. This works at the application level. That is if you use Music, then if you click on the radio icon, instead of radio buttons, you get check boxes, so you can stream to speakers attached to your Mac and then say to an old Airport Express and to an Apple TV for instance.
So if you are trying to stream a video from Music application, that’s what’s going on, the clicks are there, but it only works for the Music library not for the streaming video or music.
The problem is that this doesn’t seem to work for streaming video or audio at least on Catalina. The buttons are clicked, but only the computer thing works.
As another confusing user interface thing, there system level sound system doesn’t allow this, so you have to use an Airplay 2 aware application like Music to do this. Go figure. Nearly there!