Guys I have found this video and do not understand how to do this(0-3 seconds). I can model but I am wondering how to bring these models in Unity as the same way in this video? Have I to know scripting to do this or what? I am really curious about this.
Video:
…What video?
Sorry, a bit sleepy 
Video:
https://www.youtube.com/watch?v=8o26zE_YN3g
You you want a game in Unity that will be the ability to create a model such as this one? Look at the source code for Blender and par down it’s functionality to be applicable to a set of prefabs you have available for Unity. You can also look or modifying some publically available voxel modeling style SW code.
You have prefabbed blocks prepared. Rails, walls, stairs, etc. Then you move them into the scene and move them around till you get the result you want…
IF this was done in unity, the dude has some custom scripts in place to speedup the process, but it is pretty much the same thing.
It’s odd that the models don’t have any artifacts. Usually in Unity the seams would be visible with pixels of the background seeping through.
They won’t be visible, and pixels won’t ever seep through (I worked with modular setup a lot), unless models have errors in them.
However, you’ll see seams when you bake lighting or when you’ll have dynamic light quickly move through the environment. But no background pixels.
Build a set of Modular pieces in an external 3D modeling package. All modular pieces need to align to a grid to work properly. So 1x1=1mx1m grid. Unity standard.
Once you have crafted all the pieces needed; walls, floors, railing, stairs, etc… You will then need to export out .fbx and import into Unity.
Then set each piece up as a prefab so you can start putting pieces together in Unity like Legos.
When placing prefabs make sure you snap to the grid and use vertex snapping to make sure everything aligns properly.
Oh. It’s modular pieces. You create all grid based models and manipulate with some snap settings.
You can take a look on my environment assets on the Unity Asset Store: u3d.as/jYB
All of them are created by using grid based modular pieces and combined together like the video.
No extra plugin required but you can also use some plugins to speed up the modular workflow.
Like this one: u3d.as/kNz. This plugin is compatible with some including my modular assets.
And lets you paint modular pieces in easy way which is time saver.
If you are looking for the package used in the video: