Arc-tread geometry for a curved stair: centerline radius, walkline tread depth, and step angle.
A curved stair sweeps treads along an arc of constant radius (unlike a spiral, which wraps a tight column). Tread depth is measured along the walking line near the centerline of the stair. The step angle is the arc each tread sweeps.
A curved stair follows a large, gentle arc with two open sides and no central column; a spiral wraps tightly around a center column. Curved stairs are easier to walk but need much more space.
Along the walking line, typically near the centerline of the stair width, since the tread is wider at the outer edge and narrower at the inner edge.
Yes. The curved stringers are complex structural and joinery elements and should be engineered and shop-fabricated.