You how to thicken chili to make something called a slurry. Danilo Alfaro has published more than 800 recipes and tutorials focused on making complicated culinary techniques approachable to home cooks. The cornstarch molecules are like little sponges.
They soak up water and expand as they do so. The same thing happens with any starch. It’s the same way rice or oatmeal or polenta thicken and expand in volume when simmered. Cornstarch imparts a glossy sheen to the liquids it thickens, so it tends to be used more in sweet sauces and pie fillings than in savory sauces and gravies.
For each cup of liquid, you want to thicken, start with 1 tablespoon of cornstarch in a small bowl. Add an equal amount of cold liquid and stir until smooth paste forms. Whisk the slurry into the hot, simmering liquid that you want to thicken. Bring to a boil and simmer until any starchy taste has been cooked away. Don’t cook longer, though, as the starch may break down and the liquid will thin out again.