How to unlock a used iPhone 11 with an unknown iCloud account?

If you see a "Locked To Owner" message or one referring to Activation Locked (iCloud locked), read this Activation Lock removal article --> How to remove Activation Lock - Apple Support To remove the lock you will need either:

-The account information that set the lock (if the previous owner is deceased, read —> Request access to a deceased family member’s Apple Account - Apple Support )

OR

-The original sales receipt from Apple itself or an Apple approved vendor. Other ways you may think might be “proof” of ownership or purchase, such as having the box, or a receipt from a third party vendor such as Ebay, will likely not be acceptable by Apple. Apple may have no way of verifying the authenticity of other methods. If you have an acceptable receipt, ask Apple to unlock the device —> https://al-support.apple.com/#/additional-support

If anybody tells you that you have to be the “original” owner, they are incorrect. As long as you have the original Apple receipt (which may well not have any personally identifying information on it) you are okay.

If it is asking for an account that is not yours, take it back to the person from whom you received it. Get them to do the things in this support article or ask for a refund --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support There is no way to discover the owner of a device unless they left a message to display on the device when it is turned on, or the person who gave you the device knows them. Note that the device cannot have Lost Mode set on it. If your device has been taken over as a result of you having entered a scammer’s Account then the scammer may have set Lost Mode too and the device is lost to you.

Return or recycle the device if you cannot unlock it; it is useless to you. From the very bottom of Apple Trade In - Apple :

"Does Apple offer recycling?

Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free. That includes your batteries and old electronic products as well as free, on-demand packaging recycling for our commercial, education, and institutional customers. When we receive your device, it will be thoroughly inspected to determine if components can be recycled or reused. Whether recycled or reused, all activities relating to the processing of your device will be managed in an environmentally responsible way."

When considering future purchases read these:

The All Too Common SAD Reality of Buying … - Apple Community

If you want to buy a preowned iPhone - Apple Support

There very likely is nothing that you can do – other than to learn from this experience.

You purchased an iPhone that, at the least, was not properly prepared for sale to a new owner – an iPhone which might even be stolen property. The anti-theft lock (Activation Lock) has now kicked in, and Apple will not remove that lock on the basis of a second-hand sales receipt.

The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community