You wouldn’t wish something like that on anyone. We can only hope that this instruction will never come in handy. Nevertheless, everyone should know what to do in case of loss of an expensive device.
Go to “Settings” → “Face ID and passcode” → under “Access with screen lock” turn off the “Control Panel” and “Accessories” options. New iPhone “owners” will now find it harder to unlock the lock.
If you turn off lock screen access, no one taking possession of your iPhone will be able to view your personal information (such as an upcoming event in the Calendar widget). However, you also lose quick access to this information.
Then go to the “Apple ID” → “Locator” → “Find iPhone” settings. Turn on the “Locator Networks” and “Last Geo-Position” settings. This way you can see where your phone was last turned on in case of loss. It will be able to transmit its location for two days, even in a discharged state.