One refers to an indefinite and generic group, and it’s not a “they/them” in the sense that one does not exclude oneself from that group (it’s generic, after all). I guess universal quantification is close in meaning.

It’s a thing specific to English, or I guess Indo-European languages in general. All languages have first, second, and third person anything beyond that is non-standard. E.g. Finnish has a 0th person, “Infer who is meant from context”.