1. You can choose two different ways to stream to more than one speaker with Airplay: you either stream to multiple speakers from your iPhone screen, or you stream to one speaker and group that with others from the Sonos app. The latter seems a bit more stable to me.
2. As far a I know you cannot group Sonos speakers to a named group. In Sonos terms a speaker (or a stereo or surround set of speakers) are a “Room”. You can group “Rooms”, to have the same music from all “Rooms”. If a speaker is part of a (stereo or surround) set, you can Airplay to the “Room”, but not to a separate speaker whithin the “ Room”. The Sonos way of presenting a room differs from the Google way of presenting rooms.
3. Airplay 2 makes the speakers fetch the music from where it is stored themselves (original AirPlay made use of you iPhone for this, whence no simultanious streams). I think Airplay uses only one outside connection if you choose to stream to multiple speakers from the screen on your iPhone though, because if it would use multiple streams Spotify (for example) would not allow this. Grouping in Sonos also uses only one connection.

I am an Apple refusnik, so it may take someone with better knowledge of Airplay than me to answer this definitively, but I would have thought the following is true (the numbering doesn’t correspond to yours):

1. I am not sure what you mean by the ‘Sonos network’ in this context.  Or rather, I am not sure you know what you mean by it.  SonosNet is only in operation if a Sonos speaker is wired to the network, and affects the protocols by which data is sent between Sonos devices.  So this does not exist for many Sonos users who are using Airplay, but I don’t think that is actually relevant to your question. 
2. Airplay doesn’t even know your Sonos ‘room group’ exists.  All Airplay sees is individual airplay-enabled speakers, and it will play to one or more than one of those in the same way for Sonos speakers as for any other speakers.  Named room groups are just a convenient way of grouping speakers that you often want to play together in Sonos.
3. I would guess that Airplay triggers synchronised  streams to each speaker, but I am not an expert on Airplay.  It does what it does, irrespective of Sonos.
4. When a Sonos speaker receives an Airplay stream it will do what it does with any stream - play it on that speaker and any grouped with it.  That includes Sonos speakers that are not Airplay-compatible.
5. Exactly how the sync is handled if you choose to Airplay to two devices that are already grouped in Sonos I really don’t know.  My guess would be that it was Apple sync, but that is a guess.

So as a high-level summary: Airplay sees only individual speakers and Airplays to whichever ones you select.  When Sonos receives it it will play on the selected speakers and any currently grouped to those speakers in Sonos.

Data passing between Sonos devices will use SonosNet or WiFi depending on the Sonos mode.  Wired connections and direct routing can be used in either mode.  But I don’t think that is of particular relevance here

I hope that helps a bit.  If only to start the discussion.

And the named groups is the new S2 function found here: Kamers groeperen en groepen opheffen | Sonos I do not use it myself, 

steve9912,

I don’t think Airplay-2 always streams from source direct to a Sonos speaker - I think that’s available for Apples own HomePod speaker using Apple Music, but not for a 3rd party speaker. So if using an iPhone to stream Amazon music to a SonosNet devices off the Amazon servers using Airplay-2 for example, then it still streams via the iPhone (requesting) device, which means it needs to remain attached to the speaker network. The music will stop if you take the phone to work/shops etc. and away from the home.

Each compatible Sonos speaker ‘room’ is listed in the Airplay device list on iOS - even when they are grouped together in the Sonos App.
 

There is a toggle option in "Settings/System/Airplay” in the Sonos App to let Airplay play to the room ungrouped, or to it’s pre-existing group.

My experience is that Airplay seems work best if one Sonos Room is targeted with a single Airplay source and then if required to play that source (in sync) in other rooms to simply use the Sonos App to group together the ‘other rooms’ with the initial chosen sonos (air)player.

If you want to Airplay different audio sources to different rooms, then just send a separate Airplay stream to each separate sonos room (I would personally limit that to no more than 6 separate rooms) - a fairly good iOS App for doing that is the WHAALE multiroom player (just to mention one, of many such examples) or simply use several different Apps to stream to each Sonos room.

Thanks guys. Time to get out Wireshark and just see what’s really going on LOL.

For what it’s worth, my “Family Room” shows up as a single Airplay target from iTunes. Family Room contains an ARC and a SUB which are bonded, so I guess that makes sense. I have a few Ones I’m going to group together and do some testing.