The footer is simply a hole drilled in the ground.  A bolt ring is placed in the hole and then the hole gets filled with concrete.  The type of ground (rock, sand, etc.) and the height of the column determines how deep the footer needs to be.   Core samples are drilled and analyzed to determine the strength of the ground and the depth the footer has to be.

A ring of bolts will stick up from the concrete footer.  These bolts are used to hold the flange on the base of the column.   This is commonly used in monopole type billboard signs.