I see that older threads are marked as modified by the community moderator bot, although there are no actual edits. It brings up unanswered questions on my active list, some of which are quite interesting. However I haven't seen this happening on other sites. What's the algorithm or statistical process used for the community bot "edits"?
Those aren't edits. The community user simply 'bumps' questions without a single answer that is either upvoted or accepted to the top of the main page. As far as I know, it picks such a question at random from all of the questions meeting the criteria that exist. My impression has been that unanswered questions are 'bumped' once per month (although that isn't part of the official explanation). The community user 'bumps' one question per hour. I have no idea why you wouldn't see this on other SE sites. I assume that you would, but I don't follow any other sites closely enough to comment.
There is some additional information at this meta.SO thread: Who is the Community user?