After you hover the cursor over the particular section in the presentation, the „+” sign in the circle will appear below. You can create your own section or choose from the section templates.
Detailed description:
Click the „+” sign in the circle that will appear at the bottom of a section when you hover the cursor over it
When clicked, the „Add New Section” will appear at the top of the right toolbar
You can select the „Add Blank Section” to add a blank slide or select from the section templates