> 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.

> however you can't do multicast over the internet

Some parts of the internet do actually support multicast. The BBC did IPTV via multicast to subscribers in the UK for a while.