What is topsoilin construction?

No. Topsoil is rich in organics that decompose and compress over time, so it cannot be compacted to a stable density, and geotechnical reports and earthwork specs uniformly exclude it from fill under slabs, footings, and pavements. That is exactly why it gets stripped and stockpiled separately. It can be reused in landscape areas, berms, and lawns, where settlement does not matter and the organic content is the point.

About 12.3 cy. The math: 4 inches is 0.333 ft, so 1,000 sf x 0.333 ft = 333 cf, and 333 / 27 is about 12.3 cy. Most estimators round up for delivery, so call it 13 cy, and add more if the supplier sells by the loose yard, since material consolidates after placement and watering. The same math scales: at 6 inches the answer grows to about 18.5 cy.

Whatever the geotech borings show, which commonly runs from a few inches to a foot or more, and it often varies across the same site. The earthwork spec usually says to strip all organic material regardless of an assumed depth, so the bid quantity is an estimate with real risk attached. Smart estimators note the assumed strip depth in their bid clarifications so a deeper actual strip becomes a change rather than a loss.