As I was about to free some space from my device I noticed that 111GB of the used space are used by "System and reserved" according to the Windows 10 Storage menu.
I think it may have something to do with old windows copies as I did some resets to my computer but I can't confirm my hypothesis with my current knowledge. I used though the free up space tool from windows and nothing big comes up.
Is there some way to detect exactly what is being used and delete it?
There are many free applications that will detail exactly what is using up all the space on your computer. One of my favorites is called WinDirStat. It will also graphically show you where the "big stuff" is.
A couple strategies on this is to use the Disk Clean up app in older versions of windows (still available in newer), or storage sense to cleanup free space in Windows 10 Later builds.
One of the best utilities I have used to cleanup space was made in Australia, PatchCleaner, no cost and no link to me and it is dynamite on freeing up old unlinked patches. I am guessing if you have never run this on a machine previously and you are a regular long time patcher you will easily free up 30 to 40 GB of space - no word of a lie.