How do I sync with Apple Calendar?
The first step is to open your Apple Calendar and follow the instructions from Google in the link. https://support.google.com/calendar/answer/99358 If you have been using Apple Calendar on your iPhone you will need to go to Settings and click on Mail, Contacts, Calendars. Now click Add Account and add your Google account (you will need your Gmail account name and password). Now simply select your Gmail account from the list & make sure the calendar option is switched on.
Remember – if you continue to create appointments in your Apple Calendar, you will need to select your Google account from the dropdown on the appointment and always give your appointments an address so Ubookr can calculate travel times for you.
If you can't see the calendar on your phone or ipad, check in your google sync settings that the calendar is being synchronised with your device using this link : https://calendar.google.com/calendar/syncselect
For a detailed walk through of the process, see this how to https://www.digitaltrends.com/mobile/sync-multiple-google-calendars-on-iphone-or-ipad/