I'm not sure you understand how the Internet works. I'll explain it to you, in case you want to learn. I'm an optimist.
Caches are a symbiotic relationship between the content provider and the ISP. By streaming high-demand content more locally the load on the ISPs Internet Drains and Peering Links is dramatically reduced. Drains are typical billed per byte, so having 10s of thousands of customers each streaming the latest GoT episode can get expensive. Peering is not billed, but it still requires a serious investment in transport from the ISPs residential users back to the nearest IXP like 350 Cermak in Chicago or 56 Marietta in Dallas. Caching moves that high demand content out to the local offices so the episode only has to be downloaded once across the transport. Google does it, Netflix does it, and any ISP that isn't trying to push their own video offering embraces these caches because it reduces the load on their network without them having to invest in more transport. Anyone can do this, even the little guy by going through a 3rd party such as Akamai which sells this functionality as a service. The content isn't prioritized across the actual access network. It's not given higher CoS or dedicated infrastructure. It's just moved closer to save money and improve customer experience.
Youtube doesn't own dark fiber. Google does own dark fiber, although it's not dark because they're actually using it. This is because Google's volume of traffic is so high that it's cost effective for them to build their own transport network. This doesn't link to every ISP in the planet, it only links to major IXPs and NAPs. ISPs also come to these locations to peer and buy transit.
I can also put content up on Youtube. I'm not sure what you think this has to do with anything.