[0]: https://www.cloudflare.com/partners/peering-portal/
[1]: https://openconnect.netflix.com/en/
[2]: https://support.google.com/interconnect/answer/9058809?hl=en
Works for the ISP too, one off cost for them to drop there side of the bill down
If you are a Tier 1 ISP, everyone is willing to pay you to carry their traffic and other Tier 1s just make peering agreements with you.
If you're johnscheapvps.com, you're likely to pay all your upstream ISPs for your traffic. If you're GCP or, say, digitalocean.com, everyone would love to be paying you to get faster access to all the sites hosted on your platform (and because paying you is probably going to be cheaper than their regular upstream)
So YouTube gets more favorable terms on transit bandwidth than the random site does.
I'm not really sure how this works.
Suppose you have paying customers and for that you need X amount of bandwidth. If you add a bunch of free customers then you need X + Y bandwidth. But the price of X + Y is never going to be lower than the price of X, is it? So even if the unit cost is lower, the total cost is still higher and you haven't produced any additional revenue in exchange, so how can this produce any net profit?