To add JavaScript globally across all products in the Shopify Buy Button Channel App, access the Script Editor in your Shopify admin. Navigate to Online Store > Themes, select “Edit code” for your current theme, and locate or create the theme.js file in the Assets folder. Add your JavaScript code within a DOMContentLoaded event listener. This ensures your code executes globally. Save the changes, and your JavaScript enhancements will apply universally to all products. Test thoroughly and refer to Shopify’s documentation or support for theme-specific guidelines.