This is one of the basic techniques of glass blowing, with many variants.
At a certain temperature, glass is very easy to form.
You use this feature by pulling apart the glass lump on the glass pipe into long rods/ribbons of different widths.
Then you place these rods/ribbons on a transparent glass body. By combining rods with different colours and thickness, you can design for example, patterns that are reminiscent of tartan.