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It's the circular button at the bottom of the phone's screen. Doing so will invoke the Siri menu after a second or so; Siri will open in "listen" mode and await your instructions. If your iPhone uses AssistiveTouch for a broken Home button, tap the on-screen AssistiveTouch square and then tap Siri (or press and hold the Home icon). You can also say "Hey Siri" out-loud if you enabled the "Hey Siri" function. Once you see a multicolored line appear, you're free to begin talking to Siri. While Siri can handle most iOS-internal requests (e.g., calling a friend), Siri will search the internet for an answer to more complicated questions. It's in the bottom-left corner of Siri's screen. Doing so will bring up a list of apps with which Siri can interface and a brief description of how to use them. Doing so will close Siri. You can also say "Goodbye" if Siri is in listen mode.
Press and hold your iPhone's Home button. Wait for a rainbow line to appear at the bottom of the screen. Ask or tell Siri something. Tap ?. Press the Home button again.