Chewy and crumbly are opposite textures produced from largely the same ingredients. The variables that decide which one appears are few and mostly controllable.
Moisture is the primary lever
Chewiness requires water retained in the finished item, since chew is the resistance of a hydrated structure rather than the fracture of a dry one.
Crumbliness comes from a dry, fat-coated structure that breaks cleanly because there is nothing holding the fragments to one another.
Ingredients that hold water, including brown sugars, syrups and egg, push towards chew, while a longer bake that drives moisture off pushes towards crumb.
Fat type changes the mouthfeel directly
Fats differ in how much of them is solid at eating temperature, and a fat that is firm in the mouth breaks rather than yields.
Butter contains water as well as fat, so it contributes some moisture and some steam during baking, whereas pure fats contribute none.
Shortening-based doughs therefore tend to crumble more readily, while butter and oil doughs lean towards a softer, chewier bite at the same bake time.
Gluten development supplies the chew
Chew depends on a protein network, and that network forms when flour is hydrated and worked, which is why bread is chewy and shortbread is not.
Biscuit doughs mixed briefly with plenty of fat coating the flour develop very little network, and they crumble as a direct consequence.
Mixing longer, resting the dough so hydration completes, or using a higher-protein flour all shift the result towards chew without changing the ingredient list.
Sugar type is not interchangeable
Brown sugars retain molasses, which is hygroscopic and pulls moisture from the air into the finished biscuit over the hours after baking.
White sugar lacks that, so a biscuit made with it dries as it cools and holds a snap that a brown-sugar version loses by the next morning.
Recipes that specify a mixture are usually targeting a texture between the two rather than balancing sweetness, and the ratio is worth respecting.
Bake length and cooling decide the final state
Texture is set less by the oven temperature than by how much moisture leaves before the item comes out, which makes the last few minutes decisive.
Removing a biscuit while the centre still looks soft leaves moisture behind and produces chew, since carryover heat finishes the cooking without further drying.
Cooling on the hot tray continues that drying, so moving items to a rack early preserves chew and leaving them on it encourages crispness.