Why doesn't my iPhone Calendar sync with my iPad?

Make sure that both devices are using the same Calendar accounts. Set up multiple calendars on iPhone explains how to:

"Set up iCloud, Google, Exchange, or Yahoo calendar accounts

1. Go to Settings  > Calendar > Accounts > Add Account.
2. Do any of the following:

• • Tap a mail service (iCloud, Microsoft Exchange, or Google, for example), then sign in.
  • Tap Other, tap Add CalDAV Account or Add Subscribed Calendar, then enter your server and account information."

Exwinuse wrote:

How do I determine calendar ID my iPhone is using

Go to Settings  > Calendar > Accounts > and determine what Calendar Accounts are activated.

Asked and answered - both here and in your duplicate post.

How to link my iPhone calendar to my iPad? - Apple Community