Hello,
If a feature is not demonstrated in the theme’s live demo then it is not there.
You can customize your copy of the theme according to your own requirements.
Best regards,
Dumitru B.
I didn’t see that full width widget in demo version, but it’s avaliable in Customize with the other preset of the theme, before the 3 widgets for columns in the bottom.
You can use a third-party plugin to make widgets appear only on certain pages.
For example this: https://wordpress.org/plugins/widget-options/