Who were the founders of this city about 100 a.d.? We don´t know. Likewise, it remains a mystery why this, once the largest city in mesoamerica, went down in 650 a.d. and later was rediscovered as a ruin city by the aztec which used it again.
Here some speculations which are supported by many indications: after some catastrophic volcanic eruptions many people were driven from their settlement areas and found a new place in the valley of Teotihuacan wich almost provided paradisiacal settlement conditions. As thanks to their gods, they built those huge pyramids and temples. Neither pack animals were known nor the wheel invented, so all stones and wadding had been carried by human muscle power. the dimensions of the temples and pyramids are enormous, so, for example the sun pyramid (the name was given by the aztec) is the third largest pyramid in the world. Formerly it was thought, that the people had lived peacefully together, the numerous murals do suggest this. They practiced intensive trade and exported the valuable obsidian, which is extremly sharp if splitted skillfully. The city probably had around 200,000 inhabitants in its heyday, and new archaeological findings made clear that many animals and in addition many humans were sacrificed to the gods.
More and more humans were sacrificed (maybe because of drouts) and about 650 a.d. the residents got annoyed of the quirks of the priests and put the entire temple complex on fire. Scorch marks were found in all temples, but not in the living quarters.
The research is very complicated because the former residents knew no written language and so we even don´t know how they called themselves.
For further information go to: https://en.wikipedia.org/wiki/Teotihuacan