Atherosclerosis is initiated by high plasma cholesterol leading to monocyte entry into the artery wall and differentiation into macrophages, which take up lipoprotein cholesterol to become lipid engorged foam cells.
To me it seem like plaques form when the blood becomes «saturated» with cholesterol and it «precipitates» out in the walls of the arteries where it is eaten by immune cells which turn into foamcells when they become engorged with cholesterol.