Space-X has a large rocket factory. It's in Hawthorne, California. It looks much like an ICBM factory from the 1960s. Boca Chica is a launch and final assembly facility.
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.
The engines are built in Hawthorne, CA before being shipped to McGregor, TX to be tested then are integrated into the final assembly in Boca Chica, TX. Every other part of the launch vehicle (besides avionics systems, also manufactured and tested in Hawthorne) is manufactured and welded on site.
By mass and size, Boca Chica produces the majority of the rocket. By cost and complexity - rocket engines (of Raptor kind) and control systems (of SpaceX kind - with all kinds of telemetry channels) win from Hawthorne.