Click on an already existing section and press the ENTER key. Thus a new section is created below the existing one. You can enter text as usual as well as insert pictures and tables.
However, new inserted sections are saved as draft-sections automatically. Other users can see the new section but not edit or comment. That is very useful because you can work without distraction on your draft.
Your very own draft-sections in the document are highlighted grey. Furthermore your profile pic appears on the left margin so that you can recognize your sections at first sight. Draft-sections from other users are highlighted with a white-grey hatching and the corresponding profile pic. You can move those sections but can't edit them.
Furthermore, empty draft sections can be deleted at any time, even if they were created by other users and (still) haven't been set for review. But as soon as the editor begins to type, this is not longer possible.
If you are finished with your draft, you can set it to review by clicking on the blue arrow. After this, the section can be edited by all other users.
If you are admin and/or have the role "approver" in the document, you can also set other people's draft sections to review or even delete them at any time.