Smart Templates is the add-on for Atlassian Confluence which helps you to create dynamic content easily and reduce content duplication by allowing you to create your pages from excerpts and parts of the other pages.
Smart Templates add-on has a bundled macro called "Smart Templates" to help you with that.
Create new page and type { in the editor to bring the list of macros and start typing {smart
This will bring you the macro you are looking for
Select the macro, and the placeholder like this will be added to the editor
By default, the macro uses it's body as a template, but you can configure it to use another page or page parts
Select the macro in the editor and click Edit
A macro parameters dialog will be shown