Can anyone tell me how to make a big building(that you can go in)?

I've looked on the assets store and I found a cool office level kit, that has cubicles and hallways and the like. I want to make something like this. I'm not sure if people just mush different unity basic shapes together to form complicated shapes, or if people just add textures to cubes. It seems more complex than that, and I can't find any tutorials.
I'm trying to make a first person stealth game where you need to escape a building, so i want it to be big, modern, and capable of having objects to hide behind or in. I'm not asking anyone to make it for me, but is there any tips you can give me to make it, or tutorials you know of that are compatible with unity 5?
Thank you in advance! You can tell by my post number that I'm new at this sort of thing.

 

You use a modeling tool such as blender to create them and then import to Unity.

 

So it would be easier to use Blender to create indoor environments? I would rather stick to unity, but if it's easier to use blender to import assets than I can learn that too. Given I have prior knowledge of unity, will blender make sense to me?(like learning c++ and moving to c#)

 

Unity is not really a modeling tool (at least without addons) so you can't really compare. It's possible to create something with the basic shapes but they are mainly for prototyping. You could probably do a basic house with them but the furniture and so on would waste time when trying to put them together and you would have problems with textures. Blender was just one example and there are other tools too but blender is free and has lot of tutorials out there.

 

Thank you. The amount I don't know is amazing lol I better to get work

 

You can do it with Pro Core, even the furniture but 3D modeling software is more optimal way. I recommend Maya LT for it's ease.