I’m not honestly sure if you’re trolling, but I’ll bite anyway. Youtube is owned by Google, who is one of the world’s largest cloud providers. Note that there really is no such thing as ‘off-prem’ for Google, as they have data centers all over the planet at this point. They will actually have multiple copies of the video floating at different levels of storage and different nodes around the world, all automated to tier the data as demand rises and falls.