What two-dimensional shapes are most often associated with three-dimensional forms?
Nets
square circle
all regular polygons
Possibly triangles and rectangles.
2d forms are flat shapes
Steel investment foundries make use of ferrous shapes and forms
Yes, they are.
The lines, or how they look.
They are geometric shapes called polygons
Yes forms usually are either organic or geometric. Organic shapes resemble shapes usually met in nature; the gentle curve of a tree branch, the floral or a rock for example. Geometric shapes are easy to draw and measure with rulers and other devices. Usual geometric shapes are symmetric like circle, square, triangle, cross... for example. Geometric shapes can also be asymmetric.
It was Leonardo da Vinci who used geometric shapes to represent nature and human forms. Leonardo was an Italian polymath.