There is a folder at internal memory:
Inside it has one folder that is a big hash name (probably sha1sum), and inside that folder there are lots of files (also named as a hash something like
60b725f10c9c85c70d97880dfe8191b3.space) occupying 2.7GB of my smartphone precious internal memory.
In hexa/chars/bytes, thru X-Plore File Manager, I saw that they begin with "PK", which means they are ZIP files. So, after renaming them to .ZIP, I saw many pictures inside them that were all about games, and the manifest.xml was Uber annoying to try to read.
My conclusion, they were all unwanted, unasked, auto-downloaded .ZIP (.APK?) games.
Now I need to stop the app doing that kind of thing on my precious cellphone's memory.