The Google Assistant for iOS is now available at the Apple App Store for free. To get started, press the mic icon or start typing. Some things your Assistant — a Siri competitor — can help with:

Make quick phone calls (e.g. "Call Mom.")
Send text messages (e.g. "Text my bestie.")
Send emails (e.g. "Email your boss the latest TPS report.")
Set reminders (e.g. "Remind me to buy a birthday gift for Sarah.")
Set calendar events (e.g. "Set a calendar event for dinner with Charlie tomorrow from 7-9.")
Play music (e.g. "Play Jazz music on Youtube.")
Navigate to places (e.g. "Get me directions home.")
Ask it anything (e.g. "Will I need an umbrella today?”)

You can't replace Siri with Google Assistant. And you’ll have to open the app in order to use it.