How can I sync iMac calendar with iPhone?

Open iCloud preferences on the Mac and make sure Calendar is selected. On the iPhone go to Settings, click your name at the top and select iCloud and make sure Calendar is turned on. You'll also want to make sure that you're using the same iCloud ID for both devices.

Edit: Or what Lawrence Finch said.

On your Mac go to the Menu, System Preferences - iCloud (if you have Catalina it is in Apple ID) and check Calendars.

On your phone go to Settings/<your name>/iCloud and turn on Calendars.

Then sit back and wait and they will sync, and remain in sync as you change either one.