Research into how the aliens accomplished their terraforming is still underway.
The planet where this took place has been discovered by humans. The aliens responsible are long gone or perhaps noncommunicative in their current state. Humans are researching this planet and how it was molded, and understand the technology to varying degrees.
I could imagine landforms being molded by manipulating plate tectonics. One could manipulate plate tectonics by manipulating currents of magma driving them from beneath. You could manipulate magmatic flow by manipulating the electromagnetic forces of the planet interior. The scientist can go on to say that this is the current hypothesis of how it was done, and given the short period and huge energies involved probably done first before subsequent terraforming was done. How the aliens manipulated the electromagnetic fields of a planet core is still unknown but of course an active area of research.
He can go on to say that other aspects of the planet surface are better understood - certain mountains and one island chain are obviously composed of asteroids from the outer reaches of the system. He can note that these metallic mountains although millions of years old are still measurably sinking into the crust year by year. How they were brought in for soft landings on the planet surface is still not understood.