Like many materials, glass has different forms, depending on temperature. It can be solid, it can be liquid, and at high enough temperatures, it will break down to its constituent components and turn into a gaseous state.
I like to think of glass as being similar to water. Because of its lower temperature transitions, we can easily see water in all three states: solid, liquid, and gas. At temperatures of 32 F and below, water is a solid. At temperatures between 32 and 212 it is liquid. At temperatures above 212 it is in a gaseous state (vapor).
I’ve never understood why people have to think glass is any different.
And no, glass does not move, or at least in any time frame that humans can see. Perhaps molecular movement may occur over tens of millions of years, but if anyone ever tells you that the reason old glass is thicker at the bottom is because the glass flowed that way is full of crap. Old glass was usually poured out in molds, and the installers always put the thickest part of the pane at the bottom simply because it was easier to handle that way.