Xiaomi
Assign Google Assistant to the Power Button auf Xiaomi
On Xiaomi phones running MIUI, the Power button normally opens the power menu with restart and shutdown options. Google Assistant can only take over that button when full screen gesture navigation is active, since the shortcut relies on the extra half-second press window that gesture mode frees up.
The setting lives outside the main display options, which is why many users never find it. You reach it through Settings ➔ Additional settings ➔ Button shortcuts, where a toggle controls whether holding the Power button for 0.5 seconds launches Google Assistant.
Once enabled, a short hold wakes the Assistant directly, without opening the power menu first. Holding the button longer, or using the Power plus Volume Up combination, still brings up the regular power menu and screenshot tools, so shutting down the phone stays unaffected.
If you use three-button navigation instead of gestures, this option will not appear, since the shortcut only works with full screen gesture control enabled.