Where a file is used and what deleting does
The Used in tab in Media Manager shows where each of your files appears: posts, reels, ads and chats. One file can be used in several places and stays a single file in your library.
What deleting a file does
If a file is not used anywhere, deleting it removes it permanently. If it still appears in a post, reel, ad or chat, the file is kept so that what you published keeps working. It only leaves your library.
To take a file out of a post or an ad, edit or delete the post or ad itself. Removing the file from the library does not change what you already published.
Who can see your files
Your library and its folders are private to your account. Other people see a file only where you published or sent it.