Reset ipad
Reset ipad so that another person can use it with new id
iPad, iPadOS 15
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See-> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
Then you have to remove the iPhone from your account, see this Apple article:
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to see where you're signed in - Apple Support
Then that another person can sign in with his/her own Apple ID.