Also, 'Hey Siri' is supposedly processed locally on the phone using the Neural Engine part of Apples custom ARM chip so it should only upload voice data after you have said 'Hey Siri' or held down the home button to activate it. You can test this if you put the phone in airplane mode or go somewhere with no reception and say 'hey siri', it still recognises that you want to ask it something but just presents a message saying 'internet not available'

This is true on Pixel as well. It would be too slow otherwise.