How do I find the owner of an Iphone?

You can't. However the owner may be able to track the phone if they enabled "Find My" before they lost it, it is powered on, and has access to the Internet via either a cellular or Wi-Fi connection. Alternately, it can be "found" by other iPhones in the area.

Alternately, the owner could place the phone in "Lost Mode" and leave a means to contact them on the phone's screen.

Apple cannot help you with locating the owner, but they do provide a solution (see the 1st reference item below.) At this point, you have several other options to help out the owner. For example, depending on where you found it, they may have a "lost & found" where you can bring the phone to. Otherwise, you could take it to your local law enforcement agency.

Ref:

• Return a lost, locked iPhone - Apple Support
• Found a Lost or Stolen iPhone? Here's What to Do - MUO

Check it for contact information, post notices around where you found it, contact the police, and if applicable, the Lost & Found department of the facility involved.

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You can try a long press of the power button then say “Whose iPhone is this?” out loud. Sometimes Siri can figure it out and tell you. But not always.