I am not from the big city.

Is it possible from an engineering standpoint for a building to have a cement terrace on the 100th floor?

Most any skyscraper's structure consists of massive steel beams, on which all of the steel, concrete, and glass are supported. I believe the correct way to explain the terraces or porches is that the horizontal beams cantilever outward -- instead of ending flush with the vertical exterior. The cement also will have steel rods/mesh inserted inside for reinforcement when poured, adding to its strength.

So with the structure of beams as supports, a concrete terrace is not at all unfeasible for any floor height. Massive pilings in the earth will carry the load of the entire building.

I'm not a structural or Civil engineer so my knowledge of reinforced concrete design is very limited, but we did learn in Mechanical about "moment arms" and cantilevers.