Apple music on 5.1 AVR

About spatial audio with Dolby Atmos in Apple Music - Apple Support

Spatial audio for Apple Music is supported for Dolby Atmos supported equipment.

You would need an Apple TV 4K model. You would need a Dolby Atmos capable AV receiver, or a Dolby Atmos capable soundbar, or HomePods 2018, or AirPods, or supported Beats gear.

That being said, some AVRs may have their own virtualization settings, to re-process a stereo signal to include all speakers. (But that takes the stereo signal from the source, not the spatial audio.)

If you leave your AVR on auto output it will output surround for dolby atmos content and stereo for 2 channel content. I have an old Marantz 5.1 AVR that doesn't support dolby atmos but it does provide multi out sound. It sounds incredible. There seems to be a bug in TVos 15 currently however in that some dolby atmos music will only play from the front left speaker. Was working fine in TVOS 14 so assume Apple will fix this in a future update. Hope this helps.

Apple Music (without the Atmos option) is stereo, so 2.0.