Some lived on huge trees that survived by colliding with airborne flora, fauna, and water. Humans, descended from a failed space mission, populated the torus and gave Niven the cast he needed to make a world into a story. It was a clever and thoughtful exercise. It seemed as though the sub-genre was exhausted. But keep the gas torus in mind.
Human beings have detectable gravity fields.
Stars are only a mile across and burn out within a year of their formation. Centuries before these events, a spaceship accidentally crossed into this universe and promptly collapsed upon itself. Somehow the crew survived. They emerged into a cloud of breathable atmosphere. Ignoring the bad grammar, this seems to have rather a lot in common with Niven's ring of gas.