How do ISPs calculate throughput of backbones?

Question

Computer networks

Hello. It has always been interesting how Internet providers (or large telecommunications companies) calculate the throughput of highways that connect residential buildings. For example, if we take a typical modern "skyscraper" for 300-400 apartments, let's say each tenant has connected a 100 Mbps tariff, it turns out that the provider needs to connect a trunk with a bandwidth of 300 * 100 Mbps in order to ensure simultaneous Internet access for all residents (sounds of course crazy)?. But some providers also offer gigabit tariffs ... I heard that a highway with a much lower bandwidth is being built in the hope that all apartments will not start downloading torrents at the same time or somehow load the channel in some other way, is that true? (interested in "classic" optics scheme to the house - vitukha to the apartment) If so, how is the throughput of this highway calculated? Any mathematical methods? Or the principle - "what was, they built from it" (it can work with residential buildings, but if we need to combine data centers, where is everything according to SLA? What are the calculations for communication channels?)
PS And what can you read / watch at your leisure on the topic of building exactly operator communication networks?

Answer 1

1. not all subscribers load the channel at the same time by 100%. This is a practice proven rule.
2. The provider guarantees a link of X bytes to the uplink, and not that this link will be able to pump X bytes.
in general - the usual marketing bullshit of varying degrees of lousyness, which suits both parties.