As shown on my screenshot, a lot (but bafflingly, not all) of my .epub file thumbnails have little zip icons in the top left. I'd like those to not be there, but despite messing around in Preferences -> Thumbnails and in my 7zip settings, and clearing my thumbnail cache, I could not find a way to make them go away. (I also have no idea why some epub file thumbnails do not have the icon.) They're annoying. Can anyone help, please?
I can't really answer the question, but I can tell you that an ePub file is, essentially, a .zip file containing the files that comprise the book, its metadata, cover, etc.
Do they look the same in File Explorer?
It may be put there but whichever component is generating the epub thumbnails, or maybe a tool for zip archives. (It's not part of Opus as Opus doesn't know how to turn a .epub file into a thumbnail itself.)
It is indeed exactly the same in File Explorer. This does not seem to be related to DOpus after all, thank you and my bad!