Android / iOS devices:
Short-press the Button to activate the Voice Assistant.
Nothing Phone:
The "Button" doesn't activate the Voice Assistant by default. To enable it, open the Nothing X app, go to Controls > Button, and assign either the single press or Press & hold gesture to the Voice Assistant. Once configured, the assigned gesture will activate your Voice Assistant.
For more functions, please download the Nothing X app, available for download on Google Play and iOS App Store.