Which material is glass mostly made from?

Sand - specifically silica. Melt it and cool it fast enough and the atoms do not have time to arrange themselves into a crystal, so they freeze in the disordered arrangement of a liquid. Glass is best understood not as a solid or a liquid but as a liquid that has been caught before it could organise.

Pure silica needs around 1,700 degrees to melt, which was out of reach for most of history, so soda is added to bring the melting point down and lime to stop the result dissolving in water. That soda-lime recipe is essentially what a window and a bottle are, and it has not changed much in two thousand years.

The old story that medieval cathedral windows are thicker at the bottom because the glass has flowed downhill over centuries is not true. Glass at room temperature would take longer than the age of the universe to flow visibly. The panes are uneven because of how they were made - spun or drawn by hand - and glaziers sensibly installed them with the heavy edge at the bottom.