Best answer by Airgetlam
1) The AppleTV can send an AirPlay 2 signal to any of the Sonos speakers that can accept an AirPlay 2 signal. That's the Sonos One, the PLAY:5 Gen 2, the PLAYBASE and the Beam.
2) You can choose, when you set up the second PLAY:5, to have them in a stereo pair, i.e. one left and one right, or you can choose to set the second one up as a separate "room" in the Sonos software, and group the two rooms together, so both will be playing the same stream.
3) If you get a Beam, no matter whether you connect it with HDMI-ARC or the optical connection, it does become part of the Sonos ecosystem, and any number (well, up to 32) speakers can be grouped with it.
4) Once you have a single Sonos speaker that can receive and process an AirPlay 2 signal, you can group any other Sonos speaker, of any age, with it, and get the AirPlay 2 signal across your entire Sonos network.
If I missed a question, let me know.