Modifying the Online Store channel
Shopify provides several different ways to add additional functionality to the Online Store through your app.
Using ScriptTags in your app allows you to add functionality to an Online Store page without having to manually edit any theme templates. This means that if a merchant uninstalls your app, then they won't need to edit their theme templates to revert any changes.
An application proxy is a feature that fetches and displays data on an Online Store page from an outside location that you specify. The content you add to the Online Store with proxy pages can be dynamic. You can use application proxies to display content such as image galleries, statistics, and custom forms.
Modifying theme files
Modifying theme files directly is not recommended. You should use ScriptTags or application proxies wherever possible.
If you need to add functionality the Online Store, and ScriptTags and application proxies are not an option, then you can edit a shop’s theme files using the Asset resource. Theme files Assets are individual files that make up a shop's theme. Assets can be easily added, changed or removed from a shop's theme.