Asus routers have had issues with Sonos in the past, no idea if they have been resolved by now.

I run an S2 system along with two S1 systems on the same network, never a problem. None use SonosNet, all are WiFi.

Is there a good article/s about how Sonos works?  an article that explains which device is the one that you connect to when you open the App, how is the queue of each device kept, where is the information of which devices are part of the system kept, etc.

I have a few devices that I don’t use regularly. They are in rooms that not very often you listen music in. I do switch them off when not in use and on when I need them. Sometimes when I do that, it causes problems.  Like most devices disappear and only a few show up.

Not that I’m aware of, no. That would be Sonos kind of giving away some of their “secret sauce”, I’d think. And they’ve already had to sue Google and Denon for stealing some of their patents, and it wouldn’t surprise me if there weren’t others in discussion. 

In my limited experience, running two system, it’s normally worked fine, and the only time I ran into issues was at setiup, where I found that unplugging the “other” system worked well for me, so the other system couldn’t be “seen” while I setup. 

The other thing that has helped is setting up reserved IP addresses for each speaker in my router, but that may be a “fix” for other issues. Sure doesn’t hurt anything, though. And keeps them all separate, when doing updates, which is normally when the Sonos requests a “new” IP address from the router. 

There is no formal documentation at the level you are seeking. 

With respect to disappearing devices, the controllers are impatient and often time out -- just when they need some data. Make sure that you have reserved IP addresses for all regular network clients, especially the SONOS units. This avoids lots of issues.

You cannot predict which unit will be the a controller’s “Associated Player”. It’s the first unit discovered. With respect to Groups, the original member of the Group is the Group’s “Coordinator”. All network traffic for the Group flows through the Coordinator. The Left speaker becomes the Coordinator for stereo pairs. Obviously, the soundbar is Coordinator for a surround room. The soundbar will use a direct 5GHz link to the surround speakers and SUB(s).

For players that are “missing”, discovery may take up to 30 minutes. This is beyond the patience of just about everyone.

A copy of the music library index and all critical data is stored in each player. Nothing critical is stored in the controllers. It’s a tricky bit of business when your powered down players come back online because there could have been music library updates, WiFi password changes, Room name changes, etc. that must be shared and updated -- the mostly online units should take precedence.