Turn on iCloud Photos on your iPhone
On iPhone, go to Settings
Tap your name
Tap iCloud
Tap Photos
Turn on Sync this iPhone
On your iPad, go to Settings
Tap your name
Tap iCloud
Tap Photos
Turn on Sync this iPad
Make sure both devices are signed in with the same Apple ID
Connect both devices to Wi-Fi
Keep both devices charged and connected until syncing finishes
Open the Photos app on your iPad to view the synced photos
Use AirDrop to transfer selected photos manually if needed
Use iCloud.com to upload or download photos if necessary