How does atherosclerosis start?

Multiple causes. Atherosclerosis is a complex process with diffrent risk factors. Age, htn, high cholesterol, diabetes, family history, tobacco use. The liningof blood vessel has a barrier that protects it. If something causes damage to this, than cholesterol can enter the wall of the vessel and accumulate. Such things as high blood pressue and diabetes are examples of dz that can damage the lining and promote ashd.
In childhood typical. White blood cells, mostly macrophages (called monocytes, one of 5 white blood cell types), invade into the walls of the blood vessel to remove low density lipoprotein (fat carrying protein) particles. Macrophages die if overwhelmed with LDL & cannot export their ingested fat into high density lipoprotein particles. Cholesterol is a fat molecule in all animal cell membranes. Otherwise largely agree.