In order to discover where the Documents & Data are, run thru the process of manually backing up the Brave browser bookmarks.
Back up Bookmarks - Brave iOS browser
Locate the 3-dot button - either at the top-most or bottom-most bars of a Brave iOS browser window. Given the portrait [display] mode on my iPhone, that 3-dot button is at the lower right.
Select that 3-dot button.
At the upper left of the Bookmarks window, locate the “< Back” button and click on that - and repeat until the “< Back” button disappears.
Next, at the bottom left of the Bookmarks window, locate the tiny rectangular icon with the upward-pointed arrow:
Click on that icon.
Next, click on “Export Bookmarks”
Next, click on “Save to Files”
Notice in the following screenshot, Brave iOS browser has its own “Downloads” folder that is distinct from the iOS Files “Downlaods” folder.
Here, we are ignoring / skipping BOTH such “Downloads” folders.
Here, we are going to save to the “Brave” folder – select the “Brave” folder:
Now, rename the Bookmarks file, in order to indicate the date of your backup:
Thus, you may more easily find the “Bookmarks_20220205_Sat” file. [Again, notice that you are saving to the “Brave” folder.]
Click the “Save” button.
Leave / Quit Brave iOS browser.
Now, in the main display window(s) on your iOS device, locate the Files app – select that app.
In the list of applications that may store documents and data, choose “Brave” [and, in that list of applications, each should show the number of contained items]:
Having selected the “Brave” folder, you should find the stack of stuff that is taking up so much “drive” space on your iOS device . . . and you should also find the recently created “Bookmarks_20220205_Sat” html file.
IF the only item that you find, is the “Bookmarks_20220205_Sat” html file, then look at the other listed applications among the Files browser. Somewhere, there would seem to be an iOS file system error?