Inventory is managed through the Netlify CMS on <site>/admin/ . You can use this interface to manage the set of markdown files in content/*, which the jamcommerce site then uses to generate pages for each product using Gatsby's markdown-as-web-page infrastructure.
How is inventory tracked?
We use Gocommerce to handle inventory and sales. It runs on a server, stores things in a database (sqlite, postgres, etc), and the frontend talks to it via an API. We just need to call apropriate functions in the provided javascript library. Each product has a small <script> tag that Gocommerce can read to understand which product is being sold.
How do I add or subtract products?
You can use the Netlify CMS to create and remove products. The jamcommerce webapp lets users add and subtract products in their cart via a dropdown menu. This info is stored in javascript memory and once added to the cart, backed up in local storage so users can reload the page and maintain their cart.