Some airlines do their own catering, others contract out. What seems logical, is that a lot of airlines at hubs have their own catering. Airlines who have high operations at other airports such as focus cities often tend to have the jobs contracted out.
For example, I just started work with Horizon in their Food and Beverage operation in Seattle. They have their FABs in Seattle, Portland, and Boise. They are in the process of working with Menzies in Los Angeles to cater their flights that originate out of LAX
Something of interest...
Food and Beverage does more than cater food. They provide cups, crew food, first aid kit replenishments, aircraft information cards, fun items for kids (coloring items, trading cards), toiletries for the aircraft, customs forms, just about anything that can and may be needed on just about any flight. Plus, many flights are loaded up for more than one leg.