If you want to be asked each time before opening the app:

After you do this a few times, Google learns that you want to open the app and won't ask again. When you don’t mention the app name for some requests (for example, you say only “scan my receipt”), you can open the app from the search results. Google learns some of your preferred apps Zillow: 'Show me open houses nearby on Zillow.'.TuneIn Radio: 'Open TuneIn in car mode.'.Trulia: 'Show homes for sale in Boston on Trulia.'.TripAdvisor: 'Show attractions near me on TripAdvisor.'.Threema: 'Send a Threema message to Kate.'.Telegram : 'Send a Telegram message to Mom.'.NextPlus : 'Send a message with NextPlus.'.Instacart: 'Show instacart availability.'.Flixster: 'Show me Inception on Flixster.'.This is currently available only on certain apps, and only certain phrases work. You can say 'Ok Google' to do voice actions with other apps on your device. Remove what you've taught to Google to recognize your voice on that device. Try this if Google isn’t recognizing your voice when you say 'Hey Google.' If you don't see this setting, make sure 'Hey Google' is turned on. Teach Google the sound of your voice again.

When your device is locked, you’ll need to say “Hey Google” to get personal results. If you want to get hands-free help from Google Assistant while your Android phone or tablet is locked, make sure your Assistant on lock screen setting is turned on.You can say 'Hey Google' whether your screen is on or off. You can say 'Hey Google' while you use Google Maps and Android Auto. This setting could prevent some other voice services that use hot words or wake-up commands from working. If the screen is on or the device is charging, you can say 'Hey Google' from any screen.

Learn how to change your Google app settings. Not all settings are available for all languages. Settings can vary by device and Google app version.

Hey google show me a pic