How to open the power menu using with the power button?

As a new behavior, some Android versions default to opening the Google Assistant (Google Gemini) if you hold the power button.

To get back to the previous behavior to open the power menu instead, open Settings > System > Gestures > Press and hold power button and change the option to "Power menu".

You can also always access the power menu by pulling down the notification shade twice and tapping on the power button in the lower right corner of the screen.