I have a limited understanding of how the video thing works. I think there is a black box installed locally at most ISP connection points. These are from the major streaming services like Google/YouTube and they sit inline with all if the traffic I/O. From what I recall seeing on reddit (not a primary citation worthy memory or source), people that worked on said ISP infrastructure had no idea what the black boxes actually do in full scope, but empirically, they cache the most active streaming content locally.

Speculatively, this was one of the big reasons YT changed so much in 2017 where they started focusing on promoting fewer prominent creators over an egalitarian community. They needed to promote a narrower scope that could be effectively cached. You can usually see this behavior by watching old and obscure content. It takes longer to load, change resolutions, etc., whereas on newer stuff that is popular, the content is nearly instantaneous.