So the levels ( which are assets themselves) hold actors ( which are “instantiated assets”). The Content Browser allows you to veiw the assets available to the whole project. The Outliner allows you to browse the actors in the current level.
As far as previewing - if you double-click an asset, it will open a context specific editor - which typically has a preview pane allowing you to see the asset in different preview configurations. So you usually dont need a “preview” level - unless you have specific lighting/environment needs.
Hope this helps 