For example, a cube, we can see it from one direction, but we can't see all its sides. What we can see is represented by solid lines, and what we can't see needs to be drawn by dotted lines. A square is a plane figure, and we all draw it with solid lines.
2. Because when drawing three views, we draw a plane figure (all plane figures are drawn with solid lines), that is to say, when we look at a three-dimensional figure from a certain angle, we only draw what we can see from that angle, and don't draw what we can't see! For example, four cubes are arranged in a row. If we look at them from the front, we can only see four squares. From the side, only one square can be seen (because the other cubes are hidden by the outermost cube, so they can't be seen). In the same way, it is not difficult to understand why the three views in the title draw a solid line between two small squares on the same plane!