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.