Yeast is a living fungus that eats sugar and breathes out carbon dioxide, and dough is elastic enough to trap the gas. Everything about a loaf's texture follows from those two facts.
Yeast is not a chemical. It is a single celled fungus, and a packet of dried yeast is a crowd of dormant organisms waiting for water and warmth. Give them both, along with the sugars in flour, and they begin respiring.
The output of that respiration is carbon dioxide gas and a small amount of alcohol, which mostly bakes off. On its own the gas would simply escape. What stops it is gluten: a network of proteins that forms when flour is mixed with water and worked, and which is elastic enough to stretch around a bubble instead of tearing.
So the rise is a race between gas production and gas escape. Knead too little and the gluten network is too weak to hold anything. Prove too long and the yeast exhausts its food and the structure collapses. Bake, and the heat kills the yeast, sets the gluten, and freezes whatever bubble structure existed at that moment.
This is why the same ingredients give a dense flatbread or an open crumb depending only on handling. Nothing was added. The difference is entirely in how much gas was made and how well it was held.
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