Orbi - router and satellite locations interchangeable?

Does "multi-floor" mean multiple "above ground" floors or does it mean "basement and ground level"?

You are correct that which device is in which location has no functional importance.

The most common strategy is to locate the router near to the internet modem (fiber box, or whatever).  Most of us "take the easy way out" and locate the Orbi right next to the modem rather than try to figure out how to put it in a more optimal location.

It is often useful to balance "covering everywhere there could ever be a WiFi device" with providing the maximum support for major bandwidth users.  i.e. as long as the garage door controller can get enough 2.4G signal to open or close the door, putting an Orbi closer to it doesn't add much value.  If you have four 5K streaming IP cameras around the garage, it may need better service.  So, while "covering the house", also pay attention to where the computers, TV's, internet gamers, etc. are located.