How do I get my photos from iPhone to automatically go on iPad?

All photos stored in your iPhone and iPad’s own Photo Library will synchronise with iCloud Photos if enabled. This obviously requires adequate iCloud storage - which may require additional cost to you. If you already subscribe to the iCloud+ 50GB storage tier, the next is 200GB - and beyond that, 2TB.

iCloud+ plans and pricing - Apple Support

Some other Cloud storage provider's have a “free” storage quota - these typically being limited (as Apple) to 5GB. Google offer 15GB without additional cost.

With closure of My Photo Stream, If iCloud Photos is not an acceptable solution, you’ll either need to find an acceptable alternative cloud synchronisation solution from a different provider (at cost), or manually move photos between devices using external storage media.

Connect external storage devices to iPad - Apple Support

Your iPad and iPhone Photo Libraries can be simply synchronised using iCloud Photos.

Assuming that both your iPhone and iPad are running iOS/iPadOS 16.x - and are signed-in to iCloud with the same AppleID:

Settings > [Your Name / AppleID] > iCloud > Photos > Sync This iPhone | iPad - set to ON

Additional information about iCloud Photos can be found here:

Set up and use iCloud Photos - Apple Support