Every section in the editor has a header with its name and a set of buttons:
Add a section
Click Add Content at the end of the form to add a section at the bottom. To add one at an exact spot, open the menu of an existing section and use Add Section before or Add Section after.
A dialog opens where you pick the type and the design variant from preview pictures.
Move a section
Use the and buttons to move a section up or down. The preview updates right away.
Remove a section
Open the menu and click Remove Section.
Published pages are different
On a published page, the section content is locked in the page editor. To change it, open → Edit Section (all languages). Adding a section also goes through this dialog, so the new section is created in all languages at once.