Ah, I see. In other words, both of us were both right and wrong. The truth was basically split right down the middle. The default is standard Stereo and not Spatial Audio, unless you’re using AirPods.It does not support head tracking correct, but you can go into the Apple Music settings and set spatial audio to “always” in which case it will force any pair of headphones or speakers to play the Dolby Atmos mix of a song if available.
By default though, it’s only set to “automatic” where it will only play the file when a supported audio product is connected.
For what it’s worth, it looks like you can turn off Spatial Audio entirely in Settings, or you can turn off head tracking or Spatial Audio in Control Center via a long press of the volume control when you’ve got compatible headphones on. What can you do with HomePods in terms of turning off Spatial Audio? I’ve taken a look in the Home app settings but didn’t see anything, and I’m not on my home network to be able to test.