How can one see humbnails for indesign, Illustrator, Word etc
If you can see them in Explorer then you can see them in Opus as well, provided the Shell image extraction option is turned on in Settings -> Preferences / Lister Display Modes / Thumbnails.
If you can't see thumbnails for those formats in Explorer then you need to install something which generates them. (If you only see thumbnails for some files of a given type then it's because only some of them were saved with thumbnails. I think that's an option when saving Word docs, for example.)