When you first set up the HomePods, they automatically show up as AirPlay speakers for your devices. In my case I'm primarily using my iTunes library from my iMac.
I'm just starting to try out Apple Music, where you can simply ask Siri to play whatever you want. In that case, the two speakers can be used independently. I don't know how to use Siri to play the same thing one both speakers, maybe that is not possible yet without AirPlay 2, I don't know.
Yes (sorta) and no. You can feed to multiple devices from your computer because AirPlay 2 is at least partially implemented in OSX 11 (in iTunes there's a menubar icon, use it to pull down Airplay choices and tick the boxes you want). I say "partially" because as it is currently implemented, it sounds like the current implementation only gives you two mono feeds in the same room, not two distinct stereo channels. At least I don't hear anything remotely like true stereo's 3D sound stage.
You can't feed to more than one device at a time from any mobile device because AirPlay 2 is not yet implemented in iOS.
My overall impression is that Apple still has a lot of work to do. If Apple doesn't give us genuine stereo I have a hunch that the jailbreakers are going to be working overtime.