Glad to have you join DecoNetwork. Any content added into the HTML template section shows on every page in your system. You set that content to hide/show on specific pages of your site using CSS. Each page in DecoNetwork has a unique body id tag. You can use that in your CSS to show/hide content for only specific pages.
I would also mention that if you are attempting to add content into system pages like Create, Products you can do so by adding HTML into the body content widget for that page within Website Pages.