How to I unlock my phone I just bought that has owner icloud locked

You're not mistaken. If a used iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing the account ID and password, or by carrying out the steps detailed in How to remove Activation Lock - Apple Support (as he should have done before selling it).

If the OP is unable to contact him to do this then I'm afraid he will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it, nor find the previous owner, and neither can the websites that offer to unlock it for a fee: these are scams.

The OP should if possible return it to wherever he bought it and ask for a refund as in this event the device is completely useless.

Did you buy it new from Apple? Return it.

otherwise, you’ll need the credentials of the person who set up the device

It’s my understanding that this is a lock that can’t be removed except by owner that set it up. Please advise me if I’m mistaken.