you home page is using Generate Press Sections and the WP Show Posts plugin. The masonry layout and featured image are set in the relevant WP Show Posts list.
Pages with Sections won’t allow sidebars by default. This can be done with some code.
The alternative if you wish is to remove the Sections and WP Show Posts from the home page. And then set the Home to display the latest posts (WP Dashboard > Settings > Reading) and then you can use the Customiser > Layout > Blog settings to restyle your home page.