Turning off iCloud Photos and deleting from an old device, without deleting from new phone
How do I turn off iCloud and delete photos from my old phone without affecting iCloud and photos on my new phone?
iPhone 11
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See “How do I turn off iCloud Photos on only one device?” in the article linked above.
But there’s a whole lot more …
If you’re planning to give the device to someone else or sell it, that other person needs his/her own, unique Apple ID, and the device should absolutely be thoroughly cleansed as follows before you release it:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support