If you are prone to losing things (that’s me!) make sure that Find My Phone/Find My iPad/Find My Mac is is turned on for each of your devices. To track or find a device, sign in the iCloud.com select “Find iPhone” icon, and a map will be displayed with green dots indicating active devices. (Grey dots indicate OFFLINE devices, which means that these were signed in to your account when they were last powered on and connected to the Internet.)
Select “All Devices” in the menu bar to display the list of devices connected to the current Apple ID (and associated Family Sharing devices) will be listed. Select a device from the displayed list to determine its location.
For each device signed into your iCloud account (and those belonging to people in your “Family Sharing”), you can also view approximate battery status, play a sound, put in lost mode, or erase device remotely.
** For paired devices (Apple Watch or EarPods) if “Find My Device” is turn for the iPhone to which it is paired, it is automatically turned on for the Watch or EarPods (Since the Series 1 Watch does not have GPS or cellular, the location of the paired iPhone will be shown.)
What about 2-Factor authentication? It’s tricky if you’ve lost your (only) trusted device, but you can get a verification code locally from the “Settings” on your trusted device (running MacOS or iOS). Reference Apple’s support document on Two-Factor Authentication for more info. https://support.apple.com/en-us/HT204974