I would advise against using cleanmymac. However its personal choice.
If you download DaisyDisk, it breaks down all of your files and tells you what they are in a nice graphic display which you can drill down on.
It's rather good.
have you checked in Users and accounts to make sure you are the only one?
Do you have a backup system in place?
TimeMachine or some other? Sometimes those can be snapshots.
From a lay persons point of view, the breakdown of the files is not always much use as some knowledge of what is and isn't needed/important is required
I’d need to check with my IT literate son
am I doing something wrong do you think?
Just me and my wife and both have minimal data stored.