You'll have to procure parts whenever inventory is missing, either to fulfill production builds or sales orders. In both cases, the app can automatically handle the procurement for you. This means you don't have to add missing parts and quantities into purchase lists or production plans. Instead, the app does that for you.
You can then check which parts and quantities have been procured for any given production build or sales order through the Procurement table:
Bare in mind that, even though inventory might have been procured for a specific thing, it doesn't get in any way automatically reserved for that thing as, once inventory becomes available, it might actually be used for something else that proves to be more urgent or important. The best way to know if you need to purchase or produce more parts is to look at its Stock Balance at any given time.
When cancelling a production build or sales order, any inventory that has been procured this way will also be canceled as long as that's possible (e.g. missing inventory has been planned but not ordered or produced yet).