Maybe your iPad stores original Photos on iCloud.

Settings > [your name] > iCloud Tap Photos > Choose Optimize [device] Storage --> Set up and use iCloud Photos - Apple Support

You may buy iCloud Storage. --> Buy more iCloud storage - Apple Support

Before you buy more iCloud storage see if you can manage your existing storage --> Manage your photo and video storage - Apple Support

Your photos are always available for both iPhone and iCloud. You delete it on the iPhone it is gone from iCloud and vice-versa.
To save space on your iPhone use optimized iPhone storage. Remember, if you delete in one it is deleted everywhere.

You cannot increase the storage of an iPhone.
- However, you can optimize iPhone Storage by using iCloud Storage in order to save space on your device
- If you had turned on Messages in iCloud & iCloud Photos and you delete Photos, Videos and Messages from the iPhone then it is deleted from iCloud too, and vice-versa. At the same time, it is also deleted from all the devices that use the same Apple ID.