When you work with very complex code, it could be quite hard to look up the required function in a file or navigate thought code in a file.

The Structure tool window is located in the bottom left corner and must be activated to show a file structure:

structureToolWindow.

IDEA provides amazing functionality to show a file content that is also supported by the plugin:

structureOverview

Please try all options yourself as there are many ways to set up structure view.