Spacex makes everything in one place and does it very efficiently.
Elon Musk talks about efficiency of the new Raptor Engine
The Raptor engine has three times the operating pressure of the Merlin Engine.
It has three times the thrust but is the same size.
Spacex can use the same production techniques that have been honed with Merlin.
Spacex is making 300 Merlin engines per year. Although they use 51 Raptors for the Interplanetary space transport. They could make about 6 Interstellar space transports per year without increasing the engine production rate if they switched over to Raptors.
The Oxygen and hydrogen propellant has cooled the fuel to near freezing to achieve 12% more densification.