How is phospholipid bilayer arranged? and why does it arrange that way?

It arranges as 2 layers of phospholipids where the hydrophilic(water-loving) heads face toward the outside and the hydrophobic(water-hating) tails are on the inside. They arrange like this to allow for selective permeability which is why this is what a cell's plasma membrane is made up of.

The arrangements of lipids in the membrane makes the cell semi permeable .