They have what's called Google cache in every country. In some of these countries, and in many places, ISPs are allowed to connect to the cache for free, includ

They have what's called Google cache in every country. In some of these countries, and in many places, ISPs are allowed to connect to the cache for free, including at IXs, which are basically internet exchange hubs around the world. Google has its own fiber, so they do not pay anyone for transit in many places. Additionally, because so many people watch YouTube, ISPs are motivated to connect or peer with Google's caches on their own. This arrangement means they also don't need to pay for traffic. So streaming is extremely cost-effective for YouTube, making it hard for others to compete with them on streaming costs.