The manufacturers also contract work if they don't have the staff. Say for example they need to check the soil before the Structural Engineer can plan the footings for a ride. He would need to know the PSI rating of the soil, and any local issues or codes. So the Structural Engineer would contact a local Geotechnical Engineer and team to do the tests.
The company may also need to contract work for fabrication if demands cannot be met. So the employment of the firm may be a low number, but a lot of workers are used to complete a project. At one point Arrow had "200" people working for them, but that would include all the contract work outside just the main office and fabrication shop.