You use just to say that something happened a very short time ago, or is starting to happen at the present time. For example, if you say that someone has just arrived, you mean that they arrived a very short time ago. We use the verb arrive with at or in to talk about 'coming to', 'getting to' or 'reaching' a place where a journey ends. If we see the destination as a point, we say arrive at. If we see it as a larger area, we say arrive in.