how do i make this portion building 3 top portion

Sorry Mohammed,

I do not understand what you are asking...

Try Proportional Editing to get the Curved Shape (this is not the exact shape like in the video, but you can change the Proportional Fall-off and size):

Maybe watch the Basics Course first:

https://cgcookie.com/courses/blender-3-basics-intro-to-blender

https://cgcookie.com/courses/fundamentals-of-texturing-in-blender

Then open the original Assets file and see how it's made:

But doesn't Kent go over that in the tutorial? If so just follow what he teaches on screen. If not it's probably because it's up to you to make the city as you want it. If that's the case, you can see other modeling courses if you feel you don't know enough still. Here's some links:

https://cgcookie.com/courses/blender-3-basics-intro-to-blender

https://cgcookie.com/courses/blender-mesh-modeling-bootcamp

https://cgcookie.com/courses/press-start-your-simple-first-blender-project

https://cgcookie.com/courses/rev-model-a-low-poly-muscle-car

https://cgcookie.com/courses/fundamentals-of-3d-mesh-modeling-in-blender

https://cgcookie.com/courses/pothead-create-a-hard-surface-character-in-blender

This roof texture was an experiment for creating a procedural roof tile effect. You can find the material and reference its node network in the Cubicity Asset Library. The material is called "cubicity_roof-tiles_A". Here's a screenshot of the roof tiles texture:

I didn't teach it in the course because I don't understand the math and it's a LOT of nodes to cover without actually knowing why it all works. I referenced a couple tutorials to achieve the effect, maybe this was one of them (hard to remember):

Maybe you need to set the blend mode in the material from opaque to Alpha Blend