There are a few best practices that we recommend when it comes to composable commerce. The first is to start with a headless architecture, rather than an all-in-one platform, as it will allow you the most flexibility when breaking down each unique functionality into PBCs (Packaged Business Capabilities).
You can break down the Packaged Business Capabilities into smaller components, easily swapping components in and out, as needed. For the front end, you can build highly testable React components utilizing libraries like Storybook, mock data fetching, user interaction, and more, all in a single component for a large PBC.
Take the standard eCommerce flow, for example.
You have a product page, a cart page, a checkout page, and a confirmation page. Each of these pages can be broken down into smaller components that can be swapped out as needed and should not impact one another. The Product page should not care if the cart page is using a different component for the cart summary. The checkout page should not care if the confirmation page uses a different component for the order summary. This allows you to build a site that is highly flexible and scalable.