how to find iPhone thats powered off ?

If you are unable to see the last location using iCloud.com or the Find My app, you will not be able to use those services to find your phone

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

You can only use find my when the device is powered on and connected to a network.

If it’s powered off, the only way is to backtrack and look for it with your eyes.