How do I cast to Chromecast and use my new HomePod minis for stereo sound

Hi everyone,

I've never felt like more of a Luddite than I do now.

All I want to do is cast from my MacAir to our little projector (it was a hand-me-down, very old, I can't find a brand name but it still works)

Our old speaker finally died and I was thrilled to replace it with 2 HomePods for stereo sound and they worked great from my iPhone when I tested it with YouTube Music.

However, when I tried to watch Netflix from my iPhone, I can get the video to work but my HomePod will only won't play the sound from one mini and weirdly, enough the other one will only play my YouTube music account but not one of my stations, just a random U2 song.

When I tried to use my MacAir to watch Netflix, I could get the video up but I couldn't get the HomePods to work.

Am I missing something?