Good to know:
This is different from the pages directory, where any file in pages is considered a route.
While you can colocate your project files in app you don't have to. If you prefer, you can keep them outside the app directory.
Project organization features
Next.js provides several features to help you organize your project.
Private Folders
Private folders can be created by prefixing a folder with an underscore: _folderName
This indicates the folder is a private implementation detail and should not be considered by the routing system, thereby opting the folder and all its subfolders out of routing.
Since files in the app directory can be safely colocated by default, private folders are not required for colocation. However, they can be useful for:
Separating UI logic from routing logic.
Consistently organizing internal files across a project and the Next.js ecosystem.
Sorting and grouping files in code editors.