Pregnancy question.

I am not pregnant. I'm not trying to be either. I am just curious. So, my entire life I've been told that you can get pregnant after one time, which is true for some. But it can take months or years of trying before it happens also. I know fertility can have to do with the time frame. But I have 3 nephews. The oldest, it only took 2 times before she was pregnant (at 17) then her second, about 3 years later, it took around 7 or 8 months of trying. Then, her third which was born in October last year , it took around 9 months of trying. My high school never had sex ed, so I only know from Google and my sister. All nephews are from different men. Can the dad be the factor? Or have any of you had multiple babies by the same person and had different time frames of trying before it happened? I'm extremely curious. 🤔