How can I sync data from my iPhone to my iPad?

If both use the same Apple ID, you can sync them through iCloud. Use the controls on your iPhone and iPad to indicate what types of data (Photos, Calendars, Contacts, etc.) you want to sync to iCloud (and from it, to your other device(s)).

Automatically keep your files up to date on your iPhone and computer with iCloud - Apple Support

Use iCloud on iPad - Apple Support

If you have a lot of photos and videos, you may need to purchase (rent) more iCloud storage space. You can get more iCloud space either by itself, or as part of an Apple One plan.

iCloud+ - Apple

Apple One - Apple

Another way of synchronizing your iPhone and iPad would be to connect both to the same e-mail accounts. E.g., if you have a Gmail (Google Mail) account, you can add it to Internet Accounts on both devices, and sync your Google e-mails and Google calendar entries to both devices.