No ISP actually allocates enough upstream and peering connections for every customer to use their connections at full speed at the same time, just as no cellphone provider build their network out so every customer can be on the phone at the same time.

In both setups, there are multiple levels of bottlenecks. The bottom line is that a properly configured network makes sure they can cover their usual peak usage easily, and has enough spare capacity that special events don't impact much of the customer base, and those it impact it doesn't do so much.

Consider that ISPs that offer gigabit speeds can't easily put in individual (or even paired) networking equipment that can serve thousands of people at that speed, because such equipment doesn't exist or is far too expensive to be using all over the place in the network. What you really see is that 100 people on gigabit circuits during peak usage (likely Netflix prime time in the evening) will use less than even a fiftieth or h undredth of their allocated capacity (unless you're steaming 4k all the time, and then you'll probably use about a fortieth of a gigabit sustained). As usage goes up, ISPs allocate more resources and shift traffic around to make sure they can supply for the demand. If done well, you don't ever notice.

You can also see this in highways and freeways. No highway is build to serve all the possible people that could use it at the same time, it's built to service what the number of people expected to use it in a specific time period is. This is also a good idea of what it's like when planning fails, as these works require a long lead time and a lot of accurate forecasting that's hard, so you often see vastly under-provisioned systems, and even by the time they upgrade them, they are sometimes already under-provisioned at the new size (or the time until it's under-provisioned is relatively short).