To export inventory of an item, you need to export that on each location.
1. go to the item
2. click 'Export History'
3. when the file completely exported, download the file
The file looks like this
On hand: stock on hand
On hold: item that is reserved for order
ATS: Available to sell, equal to quantity of stock on hand minus quantity that is reserved for orders
You can get inventory of an item in a location from location view.