Your product content comes from Shopify Collections and its API. You can, however, change which Shopify Collections will display on the navigation and in the search.
Individual Product pages can have custom content added to the tabs for Reviews and Video. Just go to Entries > Products and drill down to the product you'd like to change.
The Featured Products module is shared with the homepage, so this is controlled in the Globals section of the dashboard. You can add, remove, and rearrange the Featured Products in Globals > Featured.
Product categories are dynamically pulled from Shopify's "Collections". You can select which Collections are used on this website by checking them on the whitelist here: Globals > Whitelist