Long story, I'll try and keep it short. My Windows Phone (yes, I know...) came with a few MS apps preinstalled, one of which is OneDrive that I found handy for keeping files on the phone. Simply copy what I wanted into OneDrive on a computer and it miraculously appeared in OneDrive on the phone.
But I abandoned OneDrive when one day away on a trip I had no wifi and I discovered I couldn't see any of my files on the phone. My preferred method then became connecting the phone to a computer with a cable and copying the files I wanted onto the SD card.
Last night I was fiddling about with my phone, as one does, when I looked in Office, another of the preinstalled apps that I don't use, where I discovered a great long list of every file that I had ever put in OneDrive, four years-worth of them. I couldn't believe it!
Soon tiring of deleting the files from the phone one by one, it occurred to me to look on my laptop. Opening the Office app on that (it's not used there either, I prefer LibreOffice), not only were all the unwanted files there, they were triplicated, in Office, OneDrive and even OneNote. I was mortified. How dare they hang on to all this stuff that I no longer wanted and never knew was there!