In atherosclerosis, is the calcium and cholesterol contained inside or is it attached to arterial cells?

Both. Cholesterol first accumulates within cells in the arterial wall, then they die off leaving a cholesterol-rich mess. Most atherosclerotic plaques contain little calcium and many contain none; anyone who tells you otherwise is mistaken or lying. The calcium is outside the cells.
Neither. Atherosclerosis is an accumulation of white blood cells, mostly macrophages (called monocytes, one of 5 white blood cell types), which have invaded into the walls of the blood vessel to remove low density lipoprotein (fat carrying protein) particles. The macrophages may die if overwhelmed with LDL & cannot export the ingested fat into high density lipoprotein particles. Cholesterol is wrong focus.