The actual flat belt section is made of very thick tire cord (aramid/polyester) wound round and round many many times in a linear flat fashion.
The extrusion around the cords and the tooth material is a very tough nylon plastic.
The waffling on the back of the belt is to allow the relatively stiff belt to flex more easily.
Cracks form at the square bottoms of all the waffle marks over time.
Cracks are meaningless, the belt lasts until the thick cords finally give way and that takes a long long long time.
The very few that have broken spit out cleanly and did not damage the pulleys or the cases or anything else for that matter -- simply spit out on to the road.
Belts fail at over 60,000 miles -- cracks show up around 20,000 miles roughly.
You got a ways to go yet.