What really happens to ovulation when taking Plan B?

The only way Plan B is proven to work, is by delaying your ovulation, although that is not guaranteed. It may work in other ways, but they have not been proven.
If it does delay your ovulation, it is not proven for how long it delays it.

Ovulation itself only lasts a few hours. It comes at the end of your fertile days. Those fertile days, are when your cervical mucus changes in consistency, so that it can save any sperm that enters the vagina, ready for when you ovulate. Unless you are charting your fertility or using an ovulation test, you are unlikely know exactly when you are ovulating. Apps/websites are just a guide.

The only emergency contraceptive pill proven can prevent pregnancy during the 24 hours around ovulation itself, is called Ella (Ulipristal Acetate). You can see that on the Ella website. Plan B might help at that time, but it has not been proven.

If you are still within 5 days/120 hours of the unprotected sex, & want another method of emergency contraception, you could have a Paragard IUD fitted by a doctor, to try to prevent a pregnancy from implanting.