Awarding of the "Grands", "Minors", and "Mini" jackpots can be random whether you bet the minimum or max bet (although they tend to occur more frequently following a period of higher betting). Also, bonuses are predetermined but swing high and low independently. However, during general slot play, pay lines aren't random. If that were true, then spins would pay, pay, pay, not pay, pay, not pay, etc., on to infinity. But, slots pay in cycles of spins; every 3rd spin, every 6th spin, every 7th spin, every 10th spin, every 11th spin, every 20th spin, every 21st spin, most commonly. This is NOT random action, and every slot player should at least suspect that fact. 3 sets of pays every 3 spins may happen in a row, followed by 2 sets of 7 spins, and then one set of 10 spins, as the amount of each pay slowly increases over the full course of those spins. The course then finishes and the pays drop down to the minimal amount and a new course of spin sets starts (with different spin sets) as the pays slowly rise again. This pattern repeats time after time while you play with an occasional long dry spell between spin courses after you have won either a bonus or a large pay.
People mostly have trouble discerning this pattern because they mostly just throw money at a slot or become comfortable just maintaining a set betting level and passively sit waiting for pays and bonuses to come to them. Of course, the games also work hard to distract you.
However, if you really want to do more than sit back and play passively and want to truly affect game play, then you need to become experienced in seeing the spin pattern and then learn the techniques to jog and maximize their payouts. This involves knowing each slot game you play; it's high and low symbols (which are screen precursors for pays) and how to bet accordingly at spin junctions and how to tell more quickly when a slot is just teasing you and won't pay you out no matter how much money you throw at it. Getting into that is well beyond my attempt to explain here and only acquired with experience.
However, the easiest way to perceive spin sets is to minimum-bet, and by that I mean truly minimum-bet. Find a slot that allows you to bet only a penny per spin and spin a couple of hundred spins straight on it and then suddenly ratchet it up and down and play around with your bet level on that one line. See what happens and you start to get a grasp of how a slot player can actively affect a game.