As explained in the Container Overview article, typically a website can have two containers: 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 “Separate 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 and image from Colors & Backgrounds Content
You can apply boxed container for selected pages or posts from meta settings.
The width of the container depends on the Site Layout settings.