How to transfer my iPhone contacts to my iPad?

Assuming that both your iPad and iPhone are signed-in to iCloud with the same AppleID, you simply need to enable iCloud Contacts on both devices:

Settings > [Your Name / AppleID] > iCloud > [Apps Using iCloud] Show All > Contacts - set to ON

iCloud User Guide - Apple Support

you can set it up with the others data Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

but if you use icloud on both all contacts will automatically be the same on both devices