Why does bread dough stretch instead of crumbling like dry flour?
The stretch in dough is not in the flour - it gets built the moment you add water and start kneading.
Wheat flour carries two proteins, glutenin and gliadin. On their own they do nothing, but add water and work the dough and they link up into one continuous elastic web called gluten. That web is what lets dough stretch, hold its shape, and trap gas bubbles so bread rises instead of falling apart. Dry flour has no gluten yet - you build it by mixing.
Knead a ball of bread dough and it pulls into a stretchy, springy mass; stir the same flour into a cake batter and barely mix it, and it stays soft and tender - the more you work wheat flour with water, the more gluten forms.
Want chewy and stretchy (bread, pizza)? Knead more. Want soft and tender (cake, muffins)? Mix less so less gluten forms.
It explains why pizza dough is chewy and cake is tender, and why overmixing a cake batter makes it tough and rubbery.
Gluten = glue-ten: water plus kneading glues flour proteins into one stretchy web.
Learn the idea and practice English at the same time.