Found lost iPhone

If you have found a device, turn it on and see if there's a message. The owner may have done this:

Use the Find My app to locate a missing device or item - Use the Find My app to locate a missing device or item - Apple Support

"What happens if you activate Mark As Lost

...You can also display a custom message with your phone number on the Lock screen, and keep track of your device's location."

For some iPhones --> Return a lost, locked iPhone, click here --> Return a lost, locked iPhone - Apple Support

Otherwise turn it into authorities responsible for dealing with lost items.

Take it to the police. It may have been reported lost.