The company that owns the fiber, or cable, is allowed to charge a reasonable fee for managing the infrastructure. Because most of the companies owning the fibers aren't ISPs, but power companies or just infrastructure companies, the fee is normally the same for everyone.

For bandwidth, I don't know, but they all share a database that customers can use to lookup what speeds are available for their address. I think what they do it that they have trunks coming in at various locations, so that after a certain point you transit from the fiber provider to the actual ISP. My fiber provide, which I never deal with, I only interact with the ISP, have connections to 130.000 homes and businesses. At their terminus the ISP needs to be present with a connection of their own, or more frequently, one they rent from back-haul providers.

That allows me to pick from 13 different ISPs on a single fiber connections and about the same on the cable from the local cable-tv provider. Prices are pretty much identical, but then there are added services or the quality of the service where they differentiate.

A single fiber has enough bandwidth for complete nations; you aren’t going to run out of bandwidth anytime soon.

How you share bandwidth between parties depends on the setup. More often than not the end user only has one service provider at a time, thus there is neither any sharing or contention.