First, starship is not a working rocket today. It will be made to work, but that's still in the future. They are now at a stage where they can take one complete

First, starship is not a working rocket today. It will be made to work, but that's still in the future. They are now at a stage where they can take one completely empty rocket and put it in LEO, with a little bit of fuel left over for descent. Getting a fully loaded rocket in LEO, then sending another twenty rockets to refuel it in space, then powering it on a trajectory to Mars, with enough fuel left over that it can safely land on Mars, is still probably around a decade away. And then getting it to launch again from Mars and land back on the Earth is a whole other can of worms, especially if you want it to leave with living humans on board. Secondly and more importantly, we don't have the robotics knowledge to do arbitrary construction projects in this way. Digging through frozen rocky terrain in the extreme conditions of Mars is not easy, and we don't even do that autonomous here on Earth. The Mars Rovers are at the forefront of technology in this area, and they can just about move over the rocky terrain. And of course, sending people there to operate the machines without first having a shelter is not an option, they will likely die of radiation poisoning long before they finish the project - the thin metal walls of a rocket are not a good radiation insulator.