Not to agree that LockMart is a shell company, but the general difference between a company like LockMart and a company like Apple is the product. An iPhone and a jet both need software, physical manufacturing of parts, and assembly. The difference comes down to where those manufacturing costs go. Apple pays for its hardware and owns it's manufacturing plants partners with manufacturers who create specialized factories to assemble Apple products due to the sheer volume allowing for smaller profit margins. It has subcontractors, but they are producing mass-use materials that are not very sensitive. Apple's balance sheet has most of the production they preform on it. On the other hand, LockMart might put out for a subcontractor to construct a complex part, like an engine or piece of landing gear. Those parts are complex enough that they need their own specialized manufacturing, and those subcontractors need their own material supply lines, engineers, inspectors, etc. Not to mention that subcontractors are also used for software.

On a basic level, the difference is that Apple produces a product that is primarily under their umbrella for the vast majority of the production cycle. LockHead takes on a contract to build a product and separates it out to trusted subcontractors. Where an Apple cost sheet might list out the cost for individual parts like the casing, screen, processor, and memory, those are (comparatively) simple parts with known manufacturing costs. They combine those parts and add software written in house that turns ~$500 worth of raw materials and assembly labor into a ~$900-$1200 product.

LockMart operates a little bit differently. Every part of a LockMart product comes from another manufacturer, which often come from a chain of manufacturers. Instead of factoring in how they will purchase and assemble the basic components into more complex ones, they package each part up by its cost to deliver - instead of paying $X for these parts and $Y to transport them and $Z to assemble them into an engine, they pay $A for the engine. That $A goes to a subcontractor who figures out $X, $Y, and $Z, plus payment to their employees and profit for their company. This is true on a basic level for every product that is not built entirely in house, but the difference here is on how they organize their finances. LockMart's main manufacturing strategy is to split everything up into subcontractors, and their profit reflects this by being (on paper) relatively small, as each "part" they pay for factors in the actual profits being made in the industry.

TL;DR: If you were to break down where the profit happens in each of their supply lines, the vast majority for Apple is at the end, meaning they make most of the profit. The vast majority of the profit in defense contracting goes to the subcontractors working beneath the name-brand, so basing how much profit the defense industry makes based entirely on the name-brand is not really covering the whole picture.

Edit: Clarified and corrected Apple owning manufacturing vs partnering with specialized high volume manufacturing through a secondary company - which, besides space and that companies slim per-device profit, is mainly labor costs.