The most historically accurate response regarding the growth of the Mexica kingdom at Tenochtitlan is:
Their god Huitzilopochtli told them where to begin developing their new city on a spot he provided them.
According to Mexica mythology, their god Huitzilopochtli guided them to establish their city in Tenochtitlan, where they saw an eagle perched on a cactus with a snake in its beak. This vision is an integral part of their founding story and is represented on the Mexican flag today.