There are a few ways to see the structure of a file in IntelliJ IDEA.
In Commander, for Java files, the file level is omitted showing the top-level classes contained in the file.
In case, when there are no classes in the file and for non-Java files, the file itself is shown in the list.
To see the file's structure, open the file node by pressing Enter or double-clicking on it.
In CodeEditor, there is a Structure View pane displaying the structure of a currently opened file.
You may browse the contents of the file by selecting various structure nodes in the Structure View.
To navigate to the corresponding location in the source code press F4 or double-click on the node.
For convenience, there is another type of the structure view available from the editor. The popup File Structure list may be called with Ctrl-F12 (View | File Structure).