I never got into this aspect of networking, so I truly don't know what I'm talking about and wish someone will correct me, but on some level, IP does indeed have broadcast/multicast capabilities that cause the sender's egress traffic to remain independent of the number of recipients rather than being equal to the sum of recipients' ingress traffic, right? Does this only work downstream of the last router, and therefore has limited usefulness on the internet?

> IP does indeed have broadcast/multicast capabilities that cause the sender's egress traffic to remain independent of the number of recipients rather than being equal to the sum of recipients' ingress traffic, right?

Yes multicast, however you can't do multicast over the internet. In practise the technology is mainly used in production and enterprise scenarios (broadcast, signage, hotels, stadiums, etc).

Instead big streaming platforms like netflix or twich use CDN boxes installed locally at major ISPs. Also with so much hardware acceleration on modern NICs these days, it's surprisingly easy to handle Gbits of throughput for audio/video streaming.

I think you are right. Multicast is typically udp and only available on your local net if the router is configured for it. I haven't used multicast in along so I might be wrong. I remember network updates breaking it.

https://en.m.wikipedia.org/wiki/Multicast