A famous Sanskrit aphorism registered in the great Puranas says. "Yat Pinde Tat Brahmande." It can be interpreted as "All that is outside you is within you."
Durg means the fort, and Durga is the one that resides inside this fort. On the micro level, our body is like a Durg— a fortress. And in this body, the Shakti— the energy that dwells is called Durga. The entire creation is also a Durg, albeit on the macrocosmic level. And the Shakti making the universe work is Durga.
Unaware of many facets and limited by our sphere of knowledge, when we start giving meaning to the things around us, the interpretations turn out rather diminutive.
In the simplest terms, Shakti is the power that creates. You put a tiny seed in the earth, and that seed becomes a sapling in 6-8 weeks. In another 1-2 years, it turns into a full-fledged dense tree. From where did it all begin? A tiny seed. And Shakti is that intelligence that made that seed sprout and grow up into a full-bloomed tree.
So, constricting Durga to a fixed Swaroopa or specific gestures is a pretty naive understanding of a much bigger phenomenon. Durga is not merely an idol. She is that intelligence which changes a drop of semen into a living, pulsating human being. And the amazing thing is if the nose of the father is long, so is the sons'. Even the colour of parents is imbibed by their offspring. Congenital ailments are passed on. Who is relaying this information? Just x and y chromosomes? Or is there something more than that?
Even if one tries to understand the marvels and complexity of the human body medically, the intellect is bound to go crazy. For instance, if we flatten the lung tissues and spread them out on the ground, they will take up the size of a tennis court. The weight of your brain is roughly between 1.25 to 1.50 kg, and yet it houses trillions of neurons. The nervous system laid across your body is a bigger network than any LAN network in the world. It is so intricate that even an attempt to replicate a single organ is going to be a mammoth task.
This body is that factory in which you put milk, veggies, and other consumables, but only blood is manufactured! How? Can a factory put up outside ever accomplish a feat as marvellous as this? Do we even stand a chance to develop such a level of intelligence? The answer is No! This very intelligence with which the consumed meal is being converted into the blood is Shakti.
So Durga is not just an idol or piece of the picture that gets worn out after a few years, and you get another one. These are mere symbolism created by our sagacious Rishis to make the people understand the energies that are making this Samsara work. And the nine sacred nights of Shakti worshipping, honouring, remembering, and re-energising our system with that divine power and intelligence- that's Navratri.