1) Theory 1: Due to a trauma (mechanical, high blood pressure, ...) the intima is damaged. The body reacts by releasing cytokines and growth factors. Adipose cells and smooth muscle cells immigrate from the media and proliferate.
Theory 2: Macrophages of the vascular wall increasingly ingest chemically modified LDL. The vessel damage is only a part of the whole progress.
2) Due to increased deposition of fat / LDL, so-called "foam cells" develop, which cause an inflammatory reaction.
3) The inflammatory reaction causes formation of tissue-like caps and plaques. The vessel becomes more and more narrow. The increasing occlusion is fostered by plaque rupture and thrombotic reactions.
The vessel also loses elasticity and becomes brittle, so that, in the final stage, it seems completely "fossil".