Hey sorry for late response, on my desktop right now when I go to properties and check some files and games and stuff some are in D: and C: and not just one specific location I dont know if thats weird but there's that.
If you mean the documents and pictures folder under the This Pc section, all the file locations are set in C:
And to answer your last question, as a kid I didn't know when you downloaded things you could chose which drive to download it in so I think I moved my user to D: (because D: was the drive that had more storage) and that messed everything up, now I cant update, factory reset or even use some commands in command prompt.