How can I contact the original owner of an iPhone to return a lost iPhone

Provide it to police or to lost and found near where it was located. Or recycle it.

The owner has (had) the opportunity to post a lost message, and to track the device, if they wanted.

Apple is not going to reveal the owner, nor contact the owner.

You will not be able to identify the owner if they had not provided any information that would appear on the Lock Screen. Most people will not do this as it makes them a target for scammers. The original owner can still track the device though so they know exactly where it is at. That is why it is recommended to turn into a lost and found or the police.