If Siri wasn’t activated while setting up your iPhone, you can activate it by opening Settings, pressing Siri & Search and then turning it on. You’ll want to ensure Press Home/Side Button for Siri is toggled on.
Siri is your personal assistant and can be used to control certain functions on the phone by using voice commands. To access Siri, press and hold the Side button (or Home button if your device has one) for a few seconds until the Siri icon appears at the bottom of your device’s screen.
Note that on older versions of iOS, the Siri interface would cover the whole screen – but from iOS 14 onwards, the overlay takes up much less space.