Yes SpaceX's Raptor engines would be built in Hawthorne, CA and tested in McGregor, TX like the Merlin before it. But at Boca Chica, TX they are not just fitting the components into the an already completed fuselage, but welding the entire air frame together, including the pressure vessel bulkheads and stacking the stainless steel rings into Starship and Super Heavy. As far as I know Boca Chica receives stainless-steel sheet metal and turns it into the rings segments. The rings have a 9 meter diameter, so cross-country road transportation would be a problem.
