Your questions about who decides and who edits are already answered, but I just wanted to encourage you that being marked as off topic does not in and of itself mean that nobody is going to read your post, or apply careful thought and consideration to it. I've read many "OT" posts on here and just as many of them are thoughtful and well-reasoned and interesting as the on topic ones.
in reply to (OT), who decides, who adds it?
So, don't let it get you down that somebody else decided that your post wasn't on topic. Let the fact that they allowed the discussion to happen here be enough for you.
May your tongue cleave to the roof of your mouth with the force of a thousand caramels.