Need guidence how to calendar syncing in Mobile phones without any third party app

Hi,

I have done a setup of carbonio community edition on ubuntu server 22.04. All is working fine. But when I have configured my id in the gmail app of Android and iOS the emails are syncing properly but not the Calendar. And after install the DAVx⁵-Calender+CalDAV and OneCalendar app on mobile respectively the calendar syncing works properly.

What can we do so without installing any other app the calendar syncing should be possible in mobile.?
Please suggest.