why i have rusty components around/behind my brand new 4x4 renault duster's back wheels

Its light rust on components that were not painted from the factory, this is normal for unpainted parts.

I used to work in a steel mill that made various parts and thicknesses. a lot of the plate that parts like that get cut from are rusty and dirty from just sitting in the warehouse waiting to be cut. Most likely is the parts didn't get shipped to a powder coating factory just straight to the manufacturer.

Rust on these parts is rarely a concern. They are not a sheet metal to be perforated by the rust and the rust cannot make them weak enough to fail before the warranty expires (and in the general case, they are one of the last to need replacement).

That's why they leave them unpainted.

p.s. painting them is not really going to help. These are made of cast iron and are not trivial to paint, the usual result when some entusiast goes on and paints them is the paint forming tiny pockets where water and salt accumulate and accelerate the corrosion. And after a while, the paint flakes off anyway.