At the internet scale everything is largely shared. It's not correct to say that some ISPs have shared bandwidth, some do not. Most of this idea comes from early DSL says with idea of contention ratio. These days ISPs do not really look at contention ratio or configure any specific component of network with that. Most of networks are built around peak and average traffic flows. It's similar to road design. Let's say you are building a fresh 6-lane highway in city of 14L population to a larger metro, you won't say - OMG! 14L users have to share and and use 6 lane highway but it would be more around the idea - what's average and peak demand line.
Sharing is there technically all the way from last mile (shared PON), to PONs uplink to backhual in each city, peering PNI ports etc. Most of smaller ISPs are able to sell higher speed plans for lower prices because of mix of these factors:
1. It helps in marketing - they have lower brand value and hence "giving more" helps in overcoming that factor.
2. Majority of higher plans sold to masses are with assumption that people ultimately will use tiny amount of bandwidth on usual stuff (youtube, whatsapp, instagram etc). So when you know average traffic will barely be 4-5Mbps with peaks of 20-30Mbps, it doesn't matter (until a point) whether plan is 50Mbps or 200Mbps. For much higher speeds there is challenge with the GPON shared capacity.
3. Smaller ISPs in certain cases can have lower cost. If they have decent scale, they can deploy Google, Facebook, Akamai caches etc, offload 30-40% traffic easily within that city. Larger guys cannot do that because they have more of standardised deployment of these caches in larger cities. It's easy for smaller players to find real estate in area they operate as opposed to larger telcos.
This whole "shared bandwidth" is interesting thing and sometimes also used when talking of PON Vs AON. Some people have argued that AON is better becausethere's no sharing while reality is that most people selling AON say 1Gbps from a 12 port switch won't really have a backhual of 2 x 10G. In many cases it's just 1G backhual. Entire internet with packet switching is build on shared capacity. As long as any specific part of it is not congested, it's not an issue at all.
