Juno pages and posts have a simplified layout, with the option to feature left, right, no or both sidebars. You can find this in the page editing options under “Templates,” in the right sidebar below the publishing options.

To change the sidebar for archive pages (such as your blog page, or a page displaying all the posts in a particular category, or with a particular tag) go to Appearance > Customize > Extras.

To change the sidebar settings for a single Page, edit the page and look under Page Template options, in the right sidebar below your publishing options. You’ll see a dropdown list here that allows you to select a sidebar setting for that page.