How does someone get atherosclerosis?

Many factors. Genetic, diet (high fat consumption), lack of exercise, smoking, diabetes etc...
Several issues. White blood cells, invade into the walls of the blood vessel to remove low density lipoprotein (fat carrying protein) particles. Macrophages die if overwhelmed with fat & cannot export their ingested fat into high density lipoprotein particles. Elevated blood sugar & blood pressure, even mild, inactivity, smoking, avoiding (not resolving) emotional stress, getting older, being male, older, genetic.