If you want to sync with Google Calendar, please use the following method.
1. Show Google Calendar events in iPhone Calendar
Please refer to this page.
https://support.google.com/calendar/answer/99358?en=us&co=GENIE.Platform%3DiOS&hl=en
2. Select Google Calendar as sync destination with this app's iOS calendar sync function.
The calendars in the Google account added in step 1 are also displayed in the list, so select the calendar you want to synchronize.