Source of thumbnails?

[urodoc]

New user of xnview mp on win 10, installed mostly to try to view the embedded exif thumbnails. Can someone clarify how the thumbnails shown in the browser are obtained, i.e., from the exif thumbnail, generated from the image itself by xnview, or "borrowed" from windows's thumbs.db? I see the little icon on the browser image indicating the file contains a thumbnail, but it's not clear that it's displaying that embedded thumbnail. I further understand some files contain multiple sizes of thumbnails; is there a way to view each of them? Thanks.
(P.S. It was difficult finding out what the little icons on the browser images mean, eventually happened to find a faq)
(P.P.S. The forum search for the thumbnail question doesn't work, as it excludes "thumbnail" as a keyword as too common. Maybe it should allow it to search in just titles?)

[xnview]

when you have the little icon, it use embedded thumbnail. Please check Settings>Thumbnail>Misc