Step inside the bustling/vibrant/thriving factories of Shenzhen, China's/the world's/Asia's e-cigarette hub/epicenter/capital. The air is thick with the scent/smell/aroma of freshly-made/steaming/aromatic liquids/e-juices/vapor. Here, amidst rows upon rows/a labyrinth/an intricate network of machines, skilled workers churn out millions/hundreds of