These are things that it can always be assumed your PC would know about the world. It is up to you the degree to which the information is known or whether or not your character fully understands the concepts. You can also choose to be oblivious, although some of these items will be fairly hard to avoid knowing.
Some things are supposed. These are the things that some people believe are true and others believe are complete hogwash. They are rumors that haven’t been substantiated or they have, but the masses keep denying them anyway. You can go either way your heart desires on these.