How does NetSuite Calculate inventory?

We recently connected to NetSuite using ODBC so we can navigate tables and query the database. I'm having difficulty determining how NS calculates inventory levels though.

I noticed the table "Item_Location_Map" has quantity on hand which seems accurate, but I don't see a way to specify at what point in time. I figured maybe it calculates real time based on all transactional data but ran into another issue. All the details for a transaction are on the Transactions table and Tranaction_Lines except for...oddly enough...the quantity.

If I look up a say invoices and pull in the quantity field, it is always empty. Some invoices have a quantity_packed or quantity_picked value but that doesn't really help. I found "Inventory_Numbers" has quantities but only for transactions that use inventory details.