You need to create tables and forms that do the reverse, of course.   For instance, tables to record receipts: date of receipt, product ID received, units received, etc., and forms for your user to enter receipts.  Every time you have a different operation, you need a different set of tables and forms.

In my old job, we didn't wait till the goods came in before we entered receipts.  We entered our orders for the incoming goods months before we received them -- so our sales people would know in advance when goods would come in and be available to sell.  So not only we had receipt tables and forms, we also had order tables and forms.  We entered orders when we placed them, and we entered receipts when we received them.  Yes, it was kind of double work.  But if your sales people need to know things in advance, you need to do this too.  Basically, in this kind of setup, your forms and tables need to record as many things you do in your business as possible.

Yes, basically you have it correct. You'd have purchase order and purchase order details tables and reverse the process (stock in vs. stock out). I cover this in a lot more detail in my full Developer course, but that's the gist. Then there's partial shipments, which is another set of complications. :)