How do you locally back up the Boards Manager config/files?

Be interesting to know where all that is stored and if there's a way to back up and restore without connecting to the Clown.... er I mean Cloud...

It's stored in the folder shown in the Arduino IDE at the line following File > Preferences > More preferences can be edited directly in the file.

The "Additional Boards Manager URLs" you added in File > Preferences are saved in the file under that location named "preferences.txt.

You'll also need the package_index.json and package_.json.sig files for offline usage.

The actuall boards platform installations are under the "packages" subfolder.