You can control the HomePod via the music app. You can airplay to it of course but if you scroll up you can set it to the source.
View attachment 756535
Other devices can be used to select it and adjust it (within reason).
View attachment 756536
View attachment 756537
Same applies to the Mac, just switch to your HomePod in the AirPlay selection and it will show you what is playing.
View attachment 756540
View attachment 756539
Volume adjustment can be done from any device and changes are reflected on other devices. Implementation is very good IMO.