As the products are synced from a channel or imported via CSV file, you can see the inventory for the products on Listings page, which is under the Products category on the main menu.
In the tab “Sync Enabled”, you can update the inventory counts for each product defined on the page.
These numbers are saved as shown in the image below.
All the inventory counts that are changed are reflected on your sale channels.
These products can be marked disabled (which means the product definition stays but the product will not be visible for sale).