Once the integration is connected, it will be enabled for all users in your company who belong to the same Google Workspace account.
The Google Workspace Calendar integration does not affect and is not connected at all to the Time off notifications and/or the other company calendars such as new joiners/work anniversaries/birthdays.
The entire purpose of the integration is to being able to receive event invites directly in the Google Calendar.
Calendar event tasks and 1-on-1 meeting tasks sent from Bob will appear as native events, which can be adjusted and rescheduled directly from Google Calendar.
In the settings for the Google Calendar integration, why are only a certain number of employees shown?
The total number of employees included in the list are the employees who had calendar events created through the integration for them. Once the employee creates an event or an event is created for them, more employees are added to this list on the integration page.
So eventually as more and more events are created the list of employees will grow.