How do you change the volume on your HomePod?

Regardless, the device that is controlling the Apple TV should be able to control the output volume of the HomePod. I'm watching a movie right now on Apple TV, with sound routed to my HomePods, and using the Apple TV app on my iPhone. The side buttons on the iPhone control volume.

tt2

This is working for me:

Apple TV audio output --> HomePod

iPhone running Apple TV app as foreground app --> Apple TV

Side buttons control volume of audio

If it isn't working for you I suggest you power cycle the HomePods and reboot the iPhone and Apple TV, then try again.

tt2

Oh.

Use the volume buttons on your iPhone

https://support.apple.com/en-ca/HT204989

• Want to change the volume using your iOS device? Receivers with HDMI-CEC volume support let you adjust volume with the volume buttons on the side of your iOS device. You just need to plug your Apple TV directly into your receiver and not into your television. Get help with HDMI-CEC. 

What app? The home App? There isn’t one.

If you don’t want to tap the HomePod, you can ask Siri

The app doesn’t work for me to adjust volume (using iPhone buttons) on HomePod but the Siri remote does. Except it won’t work if a purchased movie has an introductory screen before you press play

To my knowledge, the Apple TV remote does not work with HomePod.

If you have an iOS device though, open Control Center and slide it up until you see the HomePod that is "Now Playing" ... then 3D touch that and you should see the controls for the HomePod's volume and playback (Play/Pause/Next).