Phase one: drying
For the first stretch, almost nothing dramatic happens and it is easy to assume nothing is going on.
What the beans are doing is absorbing heat and driving off water as steam. That is it. But it matters enormously, because the steam becomes the medium for every chemical reaction that follows. Rush this phase and the outside of the bean starts browning before the inside has lost its water, and you end up with a cup that tastes underdeveloped no matter how long you run it afterward.
Phase two: Maillard
Once enough water is gone, sugars and amino acids start reacting with each other. This is the Maillard reaction, and it is the same chemistry that browns bread crust and sears a steak.
It is where almost everything people think of as the taste of coffee gets built. Caramel, chocolate, nuts, toast. The bean turns from green to yellow to tan to brown, and the smell coming off the roaster changes completely, from something like hay to something like baking.
First crack
Steam pressure keeps building inside each bean until the structure cannot contain it any longer, and the bean fractures audibly. It sounds a lot like popcorn, and in a full drum it is loud. First crack is the landmark every roaster works around, because it is the one moment in the roast you can hear rather than infer.
Phase three: development
Everything after first crack is development, and this is where a roaster earns their money.
Stop too early and the cup is grassy and sour, because the sugars never finished what they were doing. Go too far and the origin character disappears underneath roast character, which is exactly what dark commodity coffee is. That taste is not the farm. It is the roasting.
By the time it comes out, the bean has lost about a fifth of its weight, nearly all of it water, and it has expanded. It went in green, dense and hard. It comes out brown, brittle and light.
Try this
Before you grind your next bag, look at the beans. Even colour across the whole batch means an even roast, and patchiness means something went wrong. Then smell the dry grounds and smell the brewed cup separately. The gap between those two smells is development.