Do smart wearable startups assemble the components themselves from different vendors, or do they outsource it to a third party that takes care of the entire manufacturing process?

What is the normal batch size for production/contract size?

Most of these smart wearable products contain different micro components such as OLED, Flexible PCBs etc. So its most likely that startups contract different vendors for different components and assemble the components. What are the common features of these outsourcing contract (Minimum order size etc?)