Setting up iCloud on iPhone

"Setting up iCloud on iPhone: how to my iCloud set up my phone"

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Setting Up iCloud on your iPhone:

There's lots of ways to "set up" an iPhone with iCloud. So, from a vaguer perspective, go here: Set Up iCloud on all your Devices - Apple Support

If a Singed-In Used Device:

If this is a used device, and you cannot log into it because ti is signed into another's Apple Account. If such is the case, get the user to log out of Apple on their device. Elsewise, it is useless to you. This is the site the seller should have used, prior to selling it to you: What to do Before you Sell, Give Away, or Trade in your iPhone or iPad - Apple Support. The seller should have known better.