kirkmc

With the Remote app. But if you're doing that, you might as well just control in from the Music app on your iOS device.

I have a Sonos Amp in my office; I love it. It's minimal, and works with AirPlay. I don't need anything fancy, and this suits my needs perfectly. Sonos speakers are fine, but with the Sonos Amp, you can use any speakers you want. (As you would with any amplifier.)

Assuming you haven't updated, you're still using iTunes. If you have, you're using the Music app.

Sounds like the developers didn't get their app up to date, rather than it being an actual bug related to Catalina. There are stricter security guidelines for accessing files now.

It replaces iTunes. I doubt that you'll be able to install an older version.

Other than the lack of the column browser, in the inability to display album artwork in songs view, there are no major differences.

If you start streaming music from an app, and send it to a stereo pair of HomePods from iTunes, you can then use the Remote app to control it.

No, this has not changed in Catalina. Apple told me this is not something they are going to do right away. The Music app can stream to a stereo pair of HomePods.

Yes, that is the only limitation. Only iTunes can stream to a stereo pair from a Mac.

You have been able to stream to a stereo pair from an iOS device since it got AirPlay 2 in May, 2018. https://kirkville.com/why-apple-shouldnt-have-released-the-homepod-without-airplay-2/

Here's an overview of what's changed in macOS Catalina regrading media apps (ie, the replacements for iTunes): https://kirkville.com/macos-catalina-and-the-new-media-apps-aka-after-itunes/

Why will that not be an option when you update? I stream to my HomePods either from my iPhone or iPad, or from my iMac, controlled by the Remote app.

AFAIK, the Apple TV app on smart TVs is TV only, not music. I do agree with a soundbar for a TV, unless you are really demanding regarding the sound of movies and TV shows you watch. If you get a Dolby Atmos soundbar, you'll even get faux surround sound. It beats all the wires needed for speakers around a room.

Some people are discussing keeping a version of Mojave running on a virtual machine (in Vmware Fusion or Parallels) to be able to maintain the column browser. This doesn't seem like a long-term solution to me, but it's certainly an option. I can't see too many essential features being added to the iTunes/Music app in the future. Or you could do the same with Windows in a VM, which will get updates to iTunes.

Well, companies that "sell" videos - movies and TV shows - such as Apple, Amazon, etc., say that you can download them to own them (Amazon explicits says that; Apple doesn't), though it's not really true. You are still subject to licensing conditions, and they have DRM, so you cannot do what you want with them. There are certainly some DRM-free video downloads, but only a very small minority.