To unlock the SIMulated City trophy in Cities: Skylines, you will first need to reach the Megalopolis milestone, which will require a population of 20,000 - 90,000 depending on which map you start on.
Once you reach Megalopolis, you'll be able to purchase the ninth map tile.
To unlock a new tile, hold
, choose New Areas and then click in the direction that you want to expand your city in. Once you purchase the second tile, you will unlock the trophy below: