To ensure that this search is available to you if you need it, first set up your passcode and FaceID. The simple answer is that you can't track it. It must be turned on and must be connected to the Internet. When turned off, an iPhone is just an inert lump of glass, plastic, and metal.
It's as remotely traceable as a stone would be. To locate an iPhone that's turned off, open the Find My app on another Apple device, such as a Mac, iPad, or other iPhone. iPhone users can track their lost or stolen device through the Find My network, but there's also a way to find an iPhone if it's turned off or disconnected. This is especially useful when a person loses an iPhone that runs out of battery, or if their iPhone is stolen and turned off or put into airplane mode.
It's worth noting that this feature can only be used if it's enabled on the iPhone before it's lost, as there's no way to locate an iPhone without Find My enabled. Once these options are enabled, users will see a banner when they turn off their iPhone that says “iPhone can be found after turning it off”.