It’s just whichever one you click play on most recently. The other one just auto-pauses. For certain short bursts of audio, or depending on the source of the a

It’s just whichever one you click play on most recently. The other one just auto-pauses. For certain short bursts of audio, or depending on the source of the audio (transient system sound, unprompted audio, ring tone, etc.) the paused device can auto-resume when the interrupter is done playing. For sitting at a new device starting the same “type”of audio stream perhaps you would not auto-resume the paused feed when the new feed stops. For something like Spotify, opening your MacBook should transfer the stream completely seamlessly from your iPhone as a way to save battery. The same song should never stop playing even for a millisecond (the switchover should be imperceptible) and the UI should sync the current playlist between all iCloud devices. When you open your Mac you should be able to see the Spotify that was playing on your iPhone right there and click pause if you want. It shouldn’t really be perceptible to the user “where it’s coming from”, it should just work.