Following Apple's announcement during the WWDC keynote that Spatial Audio and Lossless Audio for Apple Music would be rolling out today, the feature now appears to be going live for some users.
I have both lossless and Dolby in the settings area of the Music app. I was able to turn both on and then play using the Pro Max and hear them. Lossless sounds better then Dolby so far for me, going to plus my USB DAC into my macbook and try the high res.
In Settings > Music, turn Sound Check on, then cycle Cellular Data off and on. That immediately got Atmos to show up for me, on both my iPhone and my iPad.