how do you find your iPhone when its dead

You don’t using Find My (although if you have Send Last Location toggled on it will automatically send that info to Apple). You would have to do it the old fashion way (physically retrace your last steps and hope you can find it by searching through everything)

Where does it say that you can find your phone if it is dead? Can you provide a source or link?

Are you confusing "iPhone Findable after Power Off" with dead? Big difference there. Dead means the battery has run down completely.

We can explain how it might be possible to find the phone after it is powered off, but this will depend on a number of factors.....good battery, other iOS devices in close proximity with connection to the Internet among other things.