icloud sign in loop?

Presumably you are being prompted for your AppleID credentials - so as to satisfy the Activation Lock. You will require the AppleID and associated password that was last used to successfully activate the device - which, for the bona-fide owner that previously activated the device, should be straightforward.

You will need the previous owner’s assistance - and they will require this information:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Otherwise, here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

If you have the necessary documentation, Apple have introduced a portal though which you may be able to request release of an Activation Lock:

How to remove Activation Lock - Apple Support & start an Activation Lock support request.

For the owner of the AppleID account, recovery of the AppleID credentials should be possible - and can be recovered here:

https://iforgot.apple.com/

To recover the credentials, the owner will need access to any one of the following:

• The primary email address mailbox that corresponds with the AppleID
• Any of the secondary/recovery email addresses that should be configured for the AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with the AppleID

More information about recovery of an AppleID and associated Password:

If you forgot your Apple ID - Apple Support

If you forgot your Apple ID password - Apple Support

My iCloud password reset a password but my mobile number disconnected