The wavy and angled edges you see between sections on the Focus Plus demo pages are made with the Section separator module. In the page editor, open the Add panel, search for Section separator and drag it into its own full-width section between the two sections you want to join. The theme also ships a ready-made Section separator section under Sections that already contains the module.
Select the module and open Styles to find all of its settings: the separator type and its shape, the top and bottom colors, and the height. Match the top color to the background of the section above and the bottom color to the background of the section below, so the separator blends into both. If a gap remains above or below the separator, set the top and bottom padding of its section to zero.
The free Focus theme does not include the Section separator module; it is available in Focus Plus.