How to control my iphone volume while using an homepod ?

You cannot control the volume of the iPhone directly, but assuming you are AirPlaying to HomePod you can control the volume of HomePod with the buttons on the top, or the voice commands "Hey Siri, volume up", "Hey Siri, volume down", "Hey Siri, volume X%".

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If your device is AirPlaying to HomePod then yes, the volume controls become linked together. The controls on the phone should still work in addition to the Siri commands you can issue to the HomePod mentioned earlier.

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