When you make them a stereo pair, they essentially become one device. So airplay will go to both, Same for music played from the music app. It will break down the stereo channels, so each HomePod only gets the left or right channel.
When using Hey Siri, she will respond from one or the other, I'm not sure if it will always be the same one, or if it may change depending on which one hears you better. You can also tap the top to talk to siri on a specific one. But if you ask siri to play music, it will come out of both, no matter which one hears you.
If you want to send different music to each, you don't want them in a stereo pair. In that case each one will play mono, but you can have them in different rooms, and you can choose if you want them both to play the same music in sync with each other, or something different on each one.
stereo pairs are designed to be a semi-permanent setup. It's easy to undo, but it's time consuming and not something you want to do as part of your daily routine.
room grouping can be changed as often you want.
you can add multiple singles or stereo pairs around the house as you want. You can also mix and match full size and minis as you want, But for a stereo pair, then those 2 must be the same kind.
Another layer in the is appleTV default audio out.
There are 2 ways to get audio from the aTV to your HPs, airplay and default audio
airplay sends only stereo. and tends to turn itself off when it feels like it, shouldn't happen mid-show, but might happen between shows, and almost surly will happen when you turn the aTV off. It's easy to access from the control center to re-enable though.
With default audio, if you've got full size HPs you will get ATMOS*, with the minis you still get just stereo. This setting should maintain itself until you disable it, including over power cycles. Also when setup this way, the aTV and HPs become one device, similar to the way the 2 HPs do when you put them in stereo mode. The big downside to this, is when you want to airplay to the homepods, your actual television will more than likely turn on.
Another plus to default audio. If you've got a 2nd gen aTV 4k or newer, and your TV has an ARC port. You can send audio from apps or the tuner on your TV and any other devices connected to the TV (cable box, DVD player, etc) to the homepods.
*debatable how much true atmos you can get with only 2 speaker locations.