If you want the geometry for the lanscape including a surface pic as texture, then you can have that from google-maps or other sources:
Put youre characters or the unreal mannequin on it to scale it up to the correct size you like to have. Then:
On this you can put the buildings: either the ones made already from googe-earth and sketchup or build you’re own.
Dont even try to make a big city or even a quarter in one map, you get massive performance drop untill unplayable (maybe around 10.000 actors). Use level-streaming and to a load-test with dummy assets before you start the actual production.