Code snippets allow you to do some advanced customization to your storefront. It's also where you might setup a few of your third party marketing scripts that we don't have integrations for.
NOTE: If you're looking to modify common pages of your site and don't know how to use CSS or javascript, you will need to hire a developer.
Navigate to Customization > Advanced > Code Snippets.
NOTE: The code editor, code snippets, and custom HTML blocks do not affect native apps. To learn more about native apps customization, please check this help guide.