How do you sync an iPad to an iPhone?

Assuming that both your iPad and iPhone are signed-in to iCloud with the same AppleID, to synchronise your devices via iCloud, you must ensure that you have correctly configured both device to use iCloud:

Settings > [Your Name / AppleID] > iCloud - enable Apps and Apple Services that you wish to synchronise between devices

More information about using iCloud can be found within the iCloud User Guide:

iCloud User Guide - Apple Support

This external article may be helpful with understanding how to keep your devices synchronised:

https://www.lifewire.com/what-you-need-to-know-to-sync-iphone-to-ipad-1999222

make sure, you use on both devices the same Apple ID, and you are logged in

then on both devices go to settings, your name, iCloud

there is a list of apps, where you can switch on/off sync

make sure, you have on both devices the same apps switched on for sync