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Netflix has special servers in most ISPs data centers that cache the most popular content so you don't have to go 'all the way over the Internet' to stream your show, just the same building your traffic already goes to for internet ingress.
On top of this, Netflix has special video codecs designed to minimize bandwidth usage while preserving video quality.
Finally, yes, Netflix has massive Internet pipes for everything not cached 'near you'.
The bitrate of 4K from streaming services like Disney and Netflix is much, much lower than your UHD Blu-ray rips. They recommend having a 16-25 Mbps connection to stream 4K, but the average bitrate is even lower. It's closer to 6-8 Mbits. They just recommend a higher Internet connection because of how streaming works (small bursts of higher rates with a lot of idle time in between).
You can calculate it accurately by just downloading the movie (if the streamer lets you, like premium subscriptions do) to see the file size, and then dividing that size by the length of the movie in seconds. That will give you the average bits per second by definition. You'll be surprised how low it is, because streamers use compression, while "pure" UHD Blu-ray avoids compression to satisfy purists.
As to how much data a streamer uses, it's immense. It's a huge chunk of the data on the Internet at any given time, with estimates in 40-60% range for all the streamers in aggregate. Look into "Content Delivery Networks" (CDNs) to see how it's delivered on a global scale. It's actually very impressive.
The best quality streamer I have found is Apple TV+ which seems to range in the 25mb/s range for 4K content.
4K streams are heavily compressed. It generally ranges from 5Mbs to 25Mbs depending on factors. This is why physical media is championed so much. A full bit rate 1080p blu ray will almost always have more detail than a 4K web stream.
But most people are not paying for 4K premium content. And compression tech like AV1 will help lower the bit rate when adoption rates are better. Look up AV1 vs VP9 vs HEVC vs AVC. It's looking to be a major improvement.
Also, these large companies are not at all structured like a home network with a single egress. They are very large networks with multiple ingress/egress, multiple cache points in multiple regions, etc. to distribute the load.
First off the squash down the rates significantly.
Lets use netflix for example they will ship ISP with enough traffic open connect appliances. Each one caps out at 2x100g connections so about 200x as fast as your home plex server. They can deploy many of these as any given ISP preferably closer to end users where there is more free bandwidth.
On the smaller end they aggressively peer with pretty much anybody even down to 10g links. It's free ish bandwidth for them and the ISP so a win win.
Yes, you are missing the thousands of servers AWS has to provide for Netflix to work the way it does. You also forget the edge caches and the peering and CDN they use for the actual file to be streamed. It's using thousands of server in dozens of countries and peering via hundreds of ISP and exchanges.
Or did you think they had a few hundred servers streaming from a few 100G uplinks?
Big streaming services use distributed servers. They likely have a copy of their streaming services in each city or even in your local ISP’s data center. This saves everyone significant bandwidth costs. A few hundred TB of storage for their most demanded content is cheap money.
Your 4k stream is 4-5 times better than anything Netflix will push out. :)