Hello, thanks for submitting the question.
In general, providers are also able to enable synchronization for their own calendar only.
For the Google Calendar option to display, though, one first needs to add some information from the Google Cloud Console so that Easy!Appointments can access the API.
The synchronization always works between Easy!Appointments and a single calendar.
You cannot mix multiple calendars of a Google account.
Starting from 1.6, the configuration for Google Calendar can be done in the integrations page, which is located under the settings.