But its not shared in the same way DSL is shared. I'm on cable and, for example, if my neighbor who is also on cable, hogs the bandwidth, that is not going to affect my connection like it would if we were both on the same DSL leg.
If you want to get down to it, all internet connections are shared, even fiber. They all got back to the same POP (point of presence) - the physical location where the ISP connects you to the Internet backbone. That point is shared by all the customers in that service area. If that "pipe" is small, it may affect all the users in that service area - or at least those who have not paid extra to get a higher priority!
That depends entirely on the infrastructure used. Original ADSL and ADSL2/2+ didn't have any issues of shared bandwidth in the same sense that VDSL has, as those technologies went to the local telephone exchange and there was usually quite a fat pipe from there onward. With VDSL/VDSL2/2+ you often only run that to a local cabinet, so it theory, fewer people are sharing the same bandwidth, but often the bandwidth from the cabinet on the street to wherever that gets bundled up, often isn't enough for everyone connected to the cabinet to get full speed.
Cable on the other hand, you're most likely going to run into a junction in the same building, or at least in the same neighbourhood, but it all depends on how fat the pipe is from there onward. If you have a good ISP, it shouldn't be a huge issue, but if they cheap out, well, then you're in the same boat as above. I can't say I'm having any issues with shared bandwidth, as I get 200Mbps during any time of the day, obviously locally.
Obviously the internet as a whole is shared, but when it comes to end user experience it really matters how "nice" your ISP is. Do they throttle certain services? Do they want you to pay extra for lower ping? (Yes, that's apparently a thing in the US now) Do they have enough bandwidth for your area? Etc.
And of course, as you said, their routing agreements and where their pipe connects to the other pipes...
It seems like a lot of US ISPs in particular like to cheap out on things, oh and Australia... The two worst countries I've been to with regards to internet connectivity. Oh and Germany.
I mean, Vietnam and India beats all of those countries by a mile, which say something...