How to locate my iPhone if it’s turned off?

If you had Find My and Find My Network enabled on the phone, and the phone still has some battery charge, it will be sending out intermittent Bluetooth signals,

If another Apple device passes close enough, it might hear the signal, and help to relay the location of your phone, AirTag-style. You would then be able to see the location with Find My on another Apple device or by logging in to the site https://www.icloud.com/find/ .

Note: If the phone was stolen, not merely lost, you could give the location to the police, but it would be unwise to go to that location yourself, You might lose a lot more than your phone.

In addition to the above:

• If your iPhone or iPad was stolen - Apple Support

And as it sometimes gets asked in this context, there is no means for a user to track by IMEI or by serial number.