What a creative is, and per-placement media
"Anytime the user scrolls, they should stop and have a look."
Single image or video is the beginner default; carousel suits several products, a step-by-step story or multiple benefits. The trainer shows AI-generated examples (a shoe brand "shop drop", a SaaS "automate the busy work", a coffee brand) and recommends putting the 9:16 ratio in your image prompt. Ads Manager accepts different media for feeds, Stories and Reels and search results; the pencil icon crops, changes ratio and replaces media - a feed image cropped to a story can lose its button.
Most underperformance is creative, and shape mismatches are the easy fix.