Samsung now lets you set up your Bixby button to launch any app or task except one: Google Assistant.
This app launches Google Assistant.
Now you can follow these steps to assign this app to your Bixby button. As a result that button will launch Google Assistant:
- Go to you device's Settings
- Select "Advanced features"
- Select "Bixby key"
- Select "Double press to open Bixby" and enable "Use single press"
- Touch "Use single press"
- Select "Open app" and touch the settings icon
- Select this app: "Assistant Launcher"

Done! Now touch your Bixby button. The first time you may be asked to use Bixby or Google's Assistant. Select Google Assistant and touch "Always".