How do I merge scenes?

I opened the standard assets project in the game and what came out was the race car demo it comes stock with the unity personal version. I then went on to make a city of cubes.

Can I import the city of cubes I made? Into the race track and have the physics apply to the car and level, but then edit the city I also made?

Do I open project? Or scene? Can you do that without closing one another?