For the most part, slot bonus rounds are randomly pre-determined, that is winning the bonus and the amount you’ve won are indeed random but that win was already determined at the point the trigger lands, the bonus as you see it is just a presentation layer.
This does not apply to gamble features such as WWTBAM, in those cases each gamble on the ladder is a new random result. In this specific example, the odds as displayed to you by the “lifelines” are the actual odds the random outcome will play to.