An advert is the creative and context — i.e: a photo on an influencers Instagram story is a different advert to the same photo on a billboard.

ad would also encompass the behind the scenes stuff - analytics and the likes. It may even refer to the campaign as a whole due to AB testing.

"How did the ad we put up last month do" sort of conversations

Ad is a superset of creative

The heirarchy of terms gets really confusing here