How to access documents and data

You would delete individual files from within the app that contains those files. Otherwise, you can delete whole apps. There is no central file system in iOS like you would see on a Windows or OS X computer.

No, sorry. Data in iOS is tied to the app used with that data. For example, if these are pages files you want to clean up, you would open pages or iCloud desktop and delete them from there. If they are photo's then you'd open photos. But there is no general file system and thus nothing that can browse for files. Each apps data exists in a protected memory space specific for that app.

You can look in settings, general, storage & iCloud settings, storage, manage storage. That area will show you how much data is associated with each app on your device.

What does it say when you plug it into iTunes? In particular, how large is "other"? Other is a catch-all of all sorts of files, temp items, caches and so forth that don't fit anywhere else. It can become corrupted and build up to excessive size (anything over 3-5GB is probably a problem).

If other seems excessive, try a simple restart (press and hold home and power buttons until you get the Apple boot logo, then let go). Or you can try erase all content and settings, then restore from backup (but sometimes th backup has the corrupted data and just puts it all,back).