how to find my iphone if its off or last location known

If it is turned off, you will not be able to track it. It will only send the last location if you had that feature turned on in Settings. See this for knowing how to track your device. If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support Find My has to be turned on before the device is lost/stolen. It has to be powered on, and it has to have an internet connection. If any of that is missing, you will not be able to track the device.