Since switching to low-cost stainless steel, SpaceX design philosophy is to rapidly prototype using low-cost materials and assembly line like mass manufacturing

Since switching to low-cost stainless steel, SpaceX design philosophy is to rapidly prototype using low-cost materials and assembly line like mass manufacturing ("the machine that builds the machine"). They expect to lose many rockets along the way. They only put 3 raptor engines in there because that's the expensive part. This is in contrast to the SLS where they spend 5 years and billions of dollars designing and building one rocket that's make of break, with the second rocket launching people.