Major bulkheads and hulls could be printed in concrete mined from asteroids. A lot of the metallic internal structures could be printed from metals mined from the same asteroids.
For that matter you could mine the interior of an asteroid and use the shell as an outer hull.
“Major bulkheads and hulls could be printed in concrete....”
Where do you propose to find quicklime (Calcium oxide) on Mars required to produce the cement for the concrete, and how do you propose to keep the Martian atmosphere from decomposing the cement and concrete with its near total composition of Carbon dioxide?