A ringworld, in an extragalactic star cluster. Home, and possibly only area of settlement, for the Precursors. Around 0.7 AU in radius, orbiting-ish around the star.
Primarily earthlike terrain, with different ecosystems and biospheres at different areas kept at approximately 0.74g. Atmosphere is a argon-nitrogen mix. Terrain ranges from tall mountainous zones to large plains, to great deserts. From space, looks like a ocean with hundreds of thousands of tiny islands, but those are, in reality, large continents similar to Earth's in size. The actual ring is kept in orbit at sub-orbital speeds, which could not keep it intact normally. Thus, the ring uses advanced anti-gravity tech to keep stable. There are large foils closer to the star, each tens of thousand of kilometers long. They simulate day and night, at about a 61 hour cycle.
Varied. From deserts to mountains to plains to frozen tundras, with massive seas in between. Life is just as varied.
The whole ring is a physical anomaly by human understanding. Only the veritable Clarketech of the Precursors allows it to even exist.
Fauna & Flora
Very, very varied. The ring holds quite literally trillions of species, with quintillions of individual lifeforms. These range from all manner of life from the galaxy, many evolved on the ring. Some originally Earth-native species are on the ring. On one section for example, giant, sapient synthetic-sized arachnoid-esque creatures live near precursor structures.
Built millions of years ago. Settled sometime during that, and has had habitation since then. At some point, the Precursors underwent a great exodus, leaving only a few million on the ring, from a population of quadrillions, or more. At the ring's size, the population density dropped to near zero.