This is what's up.
Back a few thousand years ago, there was a lot more energy present in the atmosphere of the Earth.
This energy supported the life of giant mammals- mammoths, sloths, and humans, among others.
In those days, there were giant people, separately people like us, and very small people. These different people all had their own areas of the world that belonged to themselves, and other kinds of people generally wenre't allowed to enter.
All these different types of people had their own roads, bridges, infrastructure, etc etc
In different places there were certain groups or classes of normal people who were in charge of interacting with these other types of people.
Stories of people meeting trolls and having to solve riddles to cross bridges and things like that, are memories of people encourntering non-human infrastructure.
In these cases, only the classes and groups of normal people who were designated to interact with the other people and knew the 'secret passwords' were allowed to use the infrastructure of other kinds of people.