A very silly question, I know, but I just don't understand it

How the hell does ghoulification work at all??? 😭

I'm not asking for precise numbers, I'm just curious. In the series, as far as I understand, that ghoul dude (forgot his name, sorry) got hit by radiation once and majorly (the explosion he saw) and then became a ghoul eventually? Does that mean that one-time major exposure is enough? Or is it just purely at the will of the writers? 😅😅😭😭

I'm sorry if it might sound stupid, I just haven't played all the games so I might know the lore poorly