In a high-mass star, the core contracts and heats to a temperature where carbon can fuse. After the core runs out of carbon, it contracts again, until it becomes hot enough for oxygen fusion.
Heavier elements are less efficient nuclear fuel, so each cycle takes less time.
Eventually, fusion in a massive star turns the core to iron. But fusing iron into heavier elements releases no energy. Without a fusion energy source, the core cannot maintain its pressure, and it is too massive to be supported by degenerate electrons. It implodes to a size of about 50 km in a fraction of a second. The core now has enormously high density and temperature.