How can I sign into my iCloud from someone else’s phone?

Assuming thst you don’t have your own device(s) …

You’d have to use the web interface from https://icloud.com/ AND be able to retrieve your 2FA verification code from one of your “Trusted Numbers”

The EXCEPTION is that you CAN use FindMy (to find a lost device) in your friend’s browser w/o needing a verification code.

RameyR Said:

"How can I sign into my iCloud from someone else’s phone?"

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For reference...

Troubleshooting "Locked to Owner":

Is this a Used iPhone?

Request a Refund:

Seems you've been scammed, though a full assumption on my part. So, ask this person for a refund. This is the site they should have used: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support. The seller should have known better.

Learn from This:

Next time you go about purchasing a device from someone, request that they let you log into your own iCloud account on it, prior to handing over the money. If they decide to disabled by those rules you've set forth, then do not proceed with the transaction.

You'd don't need to reveal Apple ID account credentials to some else and sign in with his device , say if they don't return the iPhone to you they will become the complete account holder of your account see the articles

Keep your Apple ID secure on iPhone - Apple Support

Security and your Apple ID - Apple Support

RameyR Said:

"How can I sign into my iCloud from someone else’s phone?"

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Signing in to Anther's Device:

With their permission, of course. I'm sure you would want someone else signing into their own iCloud account on your phone.