the base of the column will sit below the bottom of the floor slab. once thats in, they will lay some drainge pipe, gravel, and pour the slab on that.
Here is an Example from the Nova Center:
http://novacenter.builthalifax.ca/
Have a Look at this post over at BuiltHalifax - It covers the basics of concrete construction.
http://halifaxbloggers.ca/builthalif...-construction/