When you use the same Apple ID on your iPhone or iPad, synchronization occurs automatically without any additional settings. However, sometimes synchronization may not work properly due to settings or delays.
In such cases, please try the following steps:
▼ When synchronization does not occur
1. Open the "Settings" app on your device (not this app, but the Settings app).
2. Tap the thick line at the top labeled "Apple ID, iCloud, Media & Purchases."
3. Tap "iCloud."
4. Tap "Show All" at the bottom of the "Apps Using iCloud" section.
5. Turn on "iCloud Calendar."
6. Open the default "Calendar" app on your device and register one event.
7. Restart your device.