How does a black hole form from a massive star?
Stellar black holes form when the pressure of the massive star is too great.
During its lifetime the massive star fuses hydrogen into helium. Over millions of years, hydrogen is exhausted and the star fuses other elements, such as helium into lithium and so on. The star keeps on fusing elements until iron is formed. Iron is too heavy of an element for the star to fuse. So there is a breakdown. The core gets compressed to an infinite small point and its density (as well its gravity) tremendously increases. Thus, a black hole is formed. It rips the star apart.