One of the tribes in my world exists among the mountains of which the tallest is almost 16,000m. A major city for the tribe exists on the mountain at around 8000m. Therefore, what internal - or external - adaptions would the body need to make to live at that height? I have already decided that these people would be taller and thinner people. Also, would that be acceptable for living in those conditions?
Your people would have adaptations similar to the Sherpa who live at high altitude in Nepal. There are numerous studies of their adaptations, here is a review of the studies and their findings. Some highlights are increased blood flow, more oxygen-carrying capacity in the blood, stronger hearts, and larger chests. A more complete summary is below: