Structured bivy sacks, called bivy shelters, have small suspension systems that expand the head area to provide some room to sit up in. This makes the confined space more comfortable fit.
The most basic bivy sacks have an opening at the head to let you breathe and allow moisture to escape. More advanced models use mesh panels to keep bugs out.
Most bivy sacks use two different fabrics to balance protection and weight. The bottom fabric is heavier for durability. The upper fabric is usually lighter and features a waterproof/breathable laminate.