Understanding campaign hierarchy is an important part of knowing all of the changes that can be made and where to find them. The campaign creation workflow is made of 3 levels:
Campaign: This is where you can select your campaign objective, campaign focus, conversion event, and select the campaign budget optimization option.
Ad Set: This is where you define:
Budget and Schedule
Targeting, including channel, cookie type, location, and more.
The Audience you would like to reach.
Inventory
Creative: This is where you can upload new creatives or select those available in your library to associate to the specific ad set.
Learn more about Creating a New Campaign.