Creating and using one
Choose New folder, give it a name, and it appears in the library ready for content.
Move items in by dragging them, or by selecting several with the checkboxes and choosing Move content. Uploading straight into an open folder saves a step.
Rename and delete from the menu next to the folder name.
Deleting a folder does not delete what is inside it. The content moves back to the root of the library. To remove content, delete the content. See Deleting content.
Folders do not affect playback
This is the thing worth being clear about. A folder is a way to find things, nothing more. Content in a folder goes into playlists exactly like content in the root, and a screen has no idea which folder anything came from.
So folders cannot be assigned to screens, and there is no such thing as playing a folder. That is what playlists and labels are for.
Choosing a structure
Group by whatever you actually search by. Site and team hold up well. Campaign holds up while the campaign runs and then leaves you with a folder nobody dares delete.
If you find yourself wanting one item in two folders, that is a sign the structure is fighting you, and search will serve you better than more folders.