● Calendars
Simply sign in with the same Apple Account on your devices and calendars sync automatically. If they don't sync, open the Settings app on your iPhone or iPad and turn on the "Calendars" item in iCloud.
To check your Apple Account, open the "Settings" app on your iPhone or iPad and tap your name (Apple Account) at the very top.
If you use Google Calendar, add your Google account in the "Settings" app under "Calendar" → "Accounts", and make sure "Calendars" is turned on.
● Syncing ToDo, memo, and diary data
This requires a paid plan: by subscribing to the Premium Service, you can sync data using iCloud Backup. Create and overwrite a backup on the device with the latest data, then restore the backup on the device you want to receive the data. The latest data will then appear on that device.