How to move side loaded books from nook to Calibre

I have several books that I side loaded onto my nook. I would like to move them into the Calibre library but don’t know how to manage that. They are not on my computer either. Not sure how to do that (plugging it in and finding them in some file????) I’m not the most techie person in the world. Lol. Any help would be greatly appreciated. Thank you.