- the thumbnails are based on intro image or on the images displayed in the article?
The module will try to find the intro image first, if no intro image exists then it will find the full text image, if no full text image exists then it will use the first image inserted in the article
- if I choose the default variation view, the related articles will be displayed right after the content (by a plugin, I guess)?
No, related module is a module, so you can place it anywhere on your page.
- can I select only articles in a specific category to display "the related articles"?
Yes you can, there is an option to select category in the module backend.