When connected to Galaxy Buds 2 Pro Watch 4, Google Assistant doesn't work in earphone controls. Bixby opens. The default assistant google is selected on the watch. Assistant is also selected in key assignments.
@cenkcirak: As far as I can see, it is only possible to assign Bixby as the assistant in the Galaxy Wearable app > Touch controls > Touch and hold. This feature is similar to Google Assistant, so you should find that you're able to issue voice commands in the same way as you would with the Google option. Alternatively, you can say 'Hey Google' and launch the Google Assistant feature in this manner.