iOS/iPadOS has a sandboxed filesystem. Each App has its own completely self-contained area of file storage – which cannot be accessed by other Apps unless the App Developer explicitly codes the App to share its private files. As such, you may not be able to find or access your Notepad+ documents using file management Apps – such as the Apple Files App.
As for backing-up your Notepad+ documents, your iPad will automatically include all locally stored files within its iCloud backup – if you have enabled this feature:
If you wish to keep an independent copy of your Notepad+ documents, you may need to individually export them – or, where available, use a share button to transfer a copy to another App. Check your Notepad+ documentation or online help for guidance.