You can have as many levels as you wish for a BOM.
It works simply: if you have a subassembly (which has a BOM and routing) and you want to use that in a product, then add the subassembly to the product’s BOM. Now you have a multi-level BOM.
Thus, each sub-level assembly has to have a stock article.
If you are making a manufacturing order for the top level, and any of the lower level assemblies are not in stock, then the manufacturing operations for the sub-level components will be nested under the same manufacturing order.