How does Outlook sync with events created in Google?

Question

I am writing a calendar integration and using the Google Calendar api and Outlook Graph api to sync calendar events. I am receiving webhook notifications as changes are made to events, so I need a way to identify that an event in Google and an event in Outlook are the same (or different) events. Theoretically, this should be done with the iCalUId.

However, when I create an event in Google that has an Outlook attendee, the event created in Outlook does not have the same iCalUId as the event in Google.

I created a work-around for this by reading the iCalUId from the attachment sent by Google to Outlook. However, the Outlook event itself still has a different iCalUId than the Google event, which makes it difficult to process future updates.

Conceptually, Outlook is able to update an event (and show it updated on the calendar) when it is updated by Google. So Outlook must be storing some reference to the Google event.

Does anyone know what that reference is and if it is accessible through the Graph API/SDK?

UPDATE Here is the code I am using to save an event in Google using the Google API/SDK:

 var service = await GoogleAuthenticationProvider.GetCalendarService(CALENDAR_CLIENT_ID, CALENDAR_CLIENT_SECRET, CALENDAR_ACCESS_SCOPES, Person.CalendarRefreshToken).ConfigureAwait(false);

        var timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById(Person.TimeZone);

        //in google, having the organizer as an attendee causes the organizer to receive notification emails from google- which we don't want, so remove him/her from the attendees
        var attendees = Event.Attendees.Where(a => a.Value != Event.OrganizerEmail);

        var tmpEvent = new Google.Apis.Calendar.v3.Data.Event
        {
            Id = ExternalID.IsNullOrEmptyExtension() ? Convert.ToString(Event.ID) : ExternalID,
            Start = new EventDateTime
            {
                DateTime = Event.StartDate,
                TimeZone = GetGoogleTimeZoneFromSystemTimeZone(timeZoneInfo.Id)
            },
            End = new EventDateTime
            {
                DateTime = Event.EndDate,
                TimeZone = GetGoogleTimeZoneFromSystemTimeZone(timeZoneInfo.Id)
            },
            Summary = Event.Title,
            Description = Event.Description,
            Attendees = attendees.Select(a => new EventAttendee
            {
                Email = a.Value,
                ResponseStatus = "accepted"
            }).ToList(),
            GuestsCanInviteOthers = false,
            Location = Event.Location,
            Recurrence = Event.RecurrenceRule.IsNullOrEmptyExtension() ? new List<string> { } : new List<string> { "RRULE:" + Event.RecurrenceRule }
        };

        var savedEvent = new Google.Apis.Calendar.v3.Data.Event { };

        if (ExternalID.IsNullOrEmptyExtension())
        {
            EventsResource.InsertRequest insertRequest = service.Events.Insert(tmpEvent, GOOGLE_PRIMARY_CALENDARID);

            insertRequest.SendUpdates = EventsResource.InsertRequest.SendUpdatesEnum.All;

            savedEvent = await insertRequest.ExecuteAsync().ConfigureAwait(false);
        }
        else
        {
            var updateRequest = service.Events.Update(tmpEvent, GOOGLE_PRIMARY_CALENDARID, ExternalID);
            updateRequest.SendUpdates = EventsResource.UpdateRequest.SendUpdatesEnum.All;

            savedEvent = await updateRequest.ExecuteAsync().ConfigureAwait(false);
        }