You choose your placements at the ad set level of your campaign. Your campaign objective will dictate which ad placements you can use.
Ad placements are grouped based on how users experience them. Here are the different placement categories:
Feeds
Stories
In-Stream
Search
Messages
In-Article
Apps
https://adespresso.com/blog/facebook-ad-placement-improve-conversions/#:~:text=You%20choose%20your%20placements%20at,on%20how%20users%20experience%20them.