Freezing food makes it last longer. What happens on a molecular level when food is going bad, is that bacterial growth occurs that makes meat, for example, change its color and smell bad. Similarly, mold grows on vegetables and bread and makes them inedible. The speed at which these processes happen is strongly related to the temperature and to the availability of liquid water to the bacteria.

When the temperature of our food is lowered in the freezer below the freezing point of water, the bacterial growth in the food is lowered too and this helps to conserve it. However, when water freezes, it forms small crystals. In the case of meat, chicken or fish the water is located in the cells of the muscular tissue. If water crystals form, the structure of many of these cells is destroyed and this can change the consistency of the meat, chicken or fish. Similarly, it can also affect the texture of vegetables and bread. The process of the formation of water crystals depends on the time that the food needs to fully freeze.

Slow freezing leads to the formations of many crystals and bigger sized crystals, whereas faster freezing leads to smaller crystals that change the texture of the food less. This means that the faster we freeze our food the less it changes its texture. Faster freezing speeds can be achieved using a colder freezer. This is the idea of flash freezing, where food is being frozen with liquid nitrogen at temperatures of -320 °F. This is the best way to minimize the crystallization of the water and change the food texture as little as possible.

When any food item is frozen, the water present in all of the cells is frozen into ice. Ice crystals do in fact form within the bulk, making the meat/chicken/fish/vegetable/bread much harder than it previously was. Freezing slows down degradation of the item by preventing the growth of bacteria or fungi (i.e. mold) through the slowing of molecular motion.

Water freezes and forms crystals. Because water crystals (a.k.a. ice) take up more volume than the same amount of water, they expand, which means that the cells that contained the water can no longer hold the ice, and so burst. This is why freezing food causes it to become mushy.

Yes, if there is enough water in the food, ice crystals will form. Ice crystals are hard on the soft stuff around them. The ice crystals push the other molecules around into different shapes and locations. That should be the same for vegetables, too.

As food is cooled, even before it freezes, all the molecules will move slower, because molecular movement depends on heat energy.

Bread doesn't seem to get many ice crystals. For storing bread, the freezer is best, and the refrigerator is worst. Bread stays fresh for a few days at room temperature, but it gets stale fast in the refrigerator. The gluten protein molecules get stiff fast in the refrigerator, and not so fast at room temperature, and slowest in the freezer.