Variables can be inserted directly into a page, into the body of a macro or - using the source code editor - into macro definitions.
Please note that the source editor often corrupts the template source, so its use is not recommended.
A damaged source of a page often cannot be repaired. You may have to delete the template and start over.
The free projectdoc blueprints typically use variables inside macro definitions. This does not cause any problems when used outside the template editor. The template editor does not allow placeholders inside macro definitions, so it will corrupt templates that use them. Therefore: do not edit blueprint templates that use placeholders with the template editor!