How to find my iPhone if it is off

might be able to see the last location before it was turned off..

otherwise, the old fashioned way.. retracing your steps

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

You cannot find it digitally if it is OFF because your lost iPhone has no power to connect to the internet and report its position. The only option left, with that in mind, is to look for it physically.

Axel F.