I lost my phone how can i track my lost device even if it is off

What to do if your phone is lost or stolen.

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

Assuming that Find My was enabled on the lost phone, the phone can be located even though it has been powered off if there is another iPhone or iPhones in close proximity to the lost phone.

Tap Settings > General > Shut Down

See the message on your phone that says.......iPhone Findable After Power Off?

This indicates that the phone is still sending an intermittent Bluetooth signal which can be picked up if there is another iPhone or iPhones in close proximity to the lost iPhone. The other iPhones pick up the Bluetooth signal and forward it to the Internet through a process known as "crowdsourcing".

Bluetooth must be enabled on the "other" phone(s) of course as well as Find My.

The phone must be turned on and connected to the internet in order to send a location signal. Report your loss to the Police...