As i tried to say, there are 2 choices: Buy the finished item from the vendor, or 'Build Assembly'.
Since you have assembly item method available I suggest you use it.
For now assume that that there is no 'transfer to another location' and all inventory is in one pool.
That is up to the point where you received the bulk box - assuming that the items have all been defined correctly including the assembly BoM
Repacking into cases can be handled by a second build assembly of a separate 'case' item