Interaction between Spotify App and Bluos App on Andriod please explain

I have multiple Bluesound speakers. To simplify there are an upstairs stereo pair and a downstairs stereo pair.

I opened the Bluos App on Andriod, made sure the upstairs stereo pair was working. Opened Spotify and selected the Upstairs stereo as source then started music on the upstairs stereo. I noticed the Bluos App had music playing from Spotify. Everything OK.

I went downstairs and turned on the downstairs stereo pair, went to Spotify and selected the downstairs stereo pair and music was playing from them. I noticed that music was also still playing on the upstairs stereo even though in Spotify I unselected it in favour of downstairs. I changed the song for the downstairs pair, then found that the downstairs pair was playing a different song than the upstairs pair. I turned off the downstairs pair and I closed down Spotify and found that the Bluos App was still playing on the upstairs pair.

This means that the Bluos App is cacheing or downloading the Spotify playlist ?

Can you please explain the relationship between Spotify and Bluos ?

In particular how Bluos calls / interacts with Spotify. Which has control etc. This will assist in me understanding what is happening when I change sources.

Also, is the interaction the same on Windows ?