By default, some shared calendars are not synced with apps, you’ll first have to enable them on Google's end.
To achieve that, go to https://calendar.google.com/calendar/syncselect and sign in to your Google account. Then select every calendar, you’d like to sync.
After a few minutes, the calendars should now appear in the calendar list within Structured. There you can add them to your day plan.
If the calendars don’t appear in the list, open the Apple Calendar app and confirm that they are available there.