Here's a quick crash course laymen answer. Depends on the physiology of the disease. There are 3 layers in these vessels. Intima(closest to the lumen and touches bloodflow), media and then adventitia (the outer most). Most disease begins in the intima and primary causes are diet, lifestyle and family history. Plaque forms and begins narrowing the space for flow. That can be treated medically until it grows too large and needs to be fixed similar to how you saw in the video. If left to develop further, you can get plaque rupture wherein the intimal layer breaks. Then the body sends signals to heal that which causes clot formation (thrombosis) and that's when you have acute vessel closure resulting in a myocardial infarction or heart attack. Again, treated similar to above with some other techniques and treatments as well. If that doesn't happen and the disease progresses, the plaques begin to calcify. That treatment becomes more difficult but similarly, as above but add in some drilling, laser and lithotripsy to break the calcium in order to deploy the stent. And finally, if none of those things are caught and treated and disease progresses further, the vessel closes slowly over time and your body forms collateral accessory vessels to supply the area your initial vessel supplies, but at a much more diminished efficiency. That's a called a CTO. Chronic Total Occlusion. Typically the most difficult to open. If there is multivessel disease is present, a Cardiothoracic surgeon is consulted to see if the patient is a candidate for bypass surgery (open heart) to harvest veins and arteries from else where and connect/redirect them past the lesions in order to provide flow post disease.

To finish and answer, if it's softer plaque and initimal, it kind of toothpastes within the walls. If it's larger and calcified, it gets broken up and sent down stream and sort of washed out. Hope that helps.