As explained in the Container Overview article, typically a website can have two containers viz: Primary Container and Secondary Container.
Primary Container is where the main content appears, whereas Secondary Container consists of the sidebar and various different widgets in the sidebar.
When you select the “Boxed Layout”; the individual containers and widgets appear in boxes.
You can find the setting under Appearance > Customize > Layout > Container
You can change the background color from Colors & Backgrounds > Base Colors
You can apply boxed container for selected pages or posts from meta settings.
The width of the container depends on the Site Layout settings.
If you have Astra Pro, you can find additional options such as background color through our Colors and Backgrounds module. In case, if you don’t have Astra Pro, you can also add a background image with Custom CSS.