They are Rings but smaller. Culture Rings are like Niven's Ringworld, and go all the way around a star, spinning around the star to simulate gravity. Culture orbitals are also ring-shaped and spinning to simulate gravity, but they're nowhere near as big (relatively speaking; still more surface area than Earth), and generally orbit a star more like a planet does. Culture Spheres are Dyson spheres.

The size of an Orbital is dictated by day length and surface "gravity" - they are arguably more elegant than Niven style Rings which need a lot of extra stuff to generate day/night cycles (shadow squares?).

Yes. One book (The Player of Games?) gave enough numbers for me to do the calculations, and it matched about 24h and 1g.

I don't think the Culture builds rings or spheres themselves, but they exist and they are aware of them. There are civilizations in the galaxy considerably more advanced than the Culture.

I believe the Culture is credited as having creating the ring in Consider Phlebas.

Do you mean Vavatch, that was scheduled for destruction? That was Culture, but an Orbital.

Vavatch was destroyed by a militarised Culture GSV but it wasn't "owned" by the Culture.

Vavatch was, IIRC (I'd have to search through) built by Culture, but since then ceded, similarly the ships that helped evacuate it

Banks' "Orbitals" are smaller rings, perhaps planet-diameter, orbiting at a distance from their star; "Rings" are Niven-Ringworld-style structures with a star in the middle. Spheres are mentioned here and there, I think, just called "spheres".