QB does not have a WIP function for assemblies, where when items are pulled to make an assembly they are reserved from sales, but not yet added as the final assembly as a stocked item
You can simulate that function with a small work around, lets say the assembly you build is called widget-finished.
rename that to widget-WIP
create a new assembly item called widget-finished and list the widget-WIP in the BOM listing.
When you start building the widget, use build assemblies for the widget-WIP item, a month later when they are complete, build the widget finished item - doing so removes the qty from the WIP widget and now you show the item ready for sale