Sandboxing significantly improves device security in significantly reducing susceptibility to malware and viruses. Because the majority of Apps can only see files that are saved on their own sandbox, malware cannot escape and infect the remaining filesystem. This is just one of the reasons that iOS/iPadOS are so resistant to malware and other security vulnerabilities.
Files can be selectively shared with other Apps - this allowing access to the material shared - and nothing else.
Some other system-level sandboxes exist for other reasons. Music downloaded to the Music App can only be accessed within the Music App. Apple has implemented this restriction to prevent unauthorised creation of material protected by copyright and Digital Rights Management (DRM).
As for guidance from Apple, there is an extensive iPad User Guide. The Guide is a rich source of frequently overlooked information about your iPad and iPadOS. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download (the Books version is available for iPadOS 16 and earlier; for some inexplicable reason, the downloadable version is not available for iPadOS 17):
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
The Tips App, installed by default on models of iPad, provides a brief introduction - and leads you to the User Guide. In case you missed it, here is a link to the Tips App:
https://apps.apple.com/gb/app/tips/id1069509450
There are some third-party File Manager Apps available from the App Store that offer more flexibility than the native Files App.
Of those available, perhaps consider Stratopherix FileBrowser - Document Manager, or its “pro version” FileBrowser Professional:
https://apps.apple.com/app/filebrowser-professional/id854618029
https://apps.apple.com/app/filebrowser-document-manager/id364738545
You might review the extensive online Manual and help files to determine suitability for your needs.
https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php