Presumably your photos and files are being synchronised via iCloud. You can configure the services and Apps that are synchronised between devices from the respective device settings.
Assuming that you wish your iPad's Photo Library to be available via iCloud - but wish to disable synchronisation with your iPhone, you simply need to disable iCloud Photos for your iPhone...
From your iPhone settings:
Settings > [Your Name / AppleID] > iCloud > Photos > Sync This iPhone - set to OFF
The stated path to the correct settings are for iOS/iPadOS 17; other versions of iOS/iPadOS may differ slightly, but will be very similar.
Should you choose to do so, you might disable iCloud Photos for your iPad and other devices. In so doing, each device will independently maintain their own Photo Library on the device.