The old pill stops the ovaries from producing an egg each month. Please source what I believe is misinformation.
Apparently, some pills do it one way, and some another. The points below are excerpted from Web MD, a by-no-means pro-life site:
https://www.webmd.com/sex/birth-control/features/birth-control-vs-abortion
"The Birth Control Process
"Birth control prevents an egg and sperm from connecting and then leading to pregnancy. There are a few ways it works:
"It stops ovulation, so you don’t release an egg. . .
"It stops fertilization, so a sperm doesn’t reach and fertilize an egg. . . by thinning the lining of your uterus, so if the egg and sperm connect, the embryo can’t stick into the lining,” Yen says. If it can’t stick to the lining, also known as implanting, it can’t get the nutrients it needs to grow."