Ejaculation on on vagina leads to pregnancy or not? What happens if a boy ejaculated near vagina opening? And a girl suffers from periods break as well.

It's ALL about probability.
The odds are low, very low, but they exist. So if you are 1 in a few thousand, you are still the 1 who got pregnant. The closer the semen gets, the higher the odds. Withdrawing before ejaculation increases the odds of pregnancy a lot, almost as much as a breaking or leaking condom. And so on.. even the pill has a tiny tiny chance of failure.

As for symptoms, there is not one single symptom that 'proves' pregnant or not pregnant, because menstrual and pregnancy symptoms are too much alike.