It’s not an ad. It’s a Siri suggestion.

Siri suggestions are just recommendations from the Siri search engine, based on your input. Since the device was new out of the box, it didn’t have any decent input yet so it just looks like a “random ad”. After using the phone for a while, the Siri suggestions will be more in tune with your habits (like your favorite traffic app for commuting, your upcoming calendar events, etc).

Apple doesn’t sell advertising space on their Home Screen.