Making sense of "slot machine bonus rounds" starts inside the cabinet. A slot machine looks like a game of reels, but what decides every payout is a random number generator working out of sight. The reels, the paylines and the buttons on the screen only show a result that was fixed the moment the spin began. Knowing how that works, from mechanical machines to modern video slots with megaways and bonus rounds, clears up most of what players wonder about them.
Paylines on a slot machine are the patterns across the reels that count as a win, from a single middle line on classic games to hundreds on modern ones. Megaways games change the number of symbols on each reel every spin, giving tens of thousands of ways to win. The buttons set the stake and the number of lines. Which type of slot machine is best depends on what a player wants: steady small wins, or large ones that come rarely.
Bonus rounds work as games inside the game: a set of symbols triggers free spins, a pick feature or a wheel, each paid from the same return to player as the base game. Slot autoplay features run a chosen number of spins without pressing the button, with limits on losses or single wins that stop it early. The simplest way to learn slot machine features before playing is the game's own help screen or a free demo.
A random number generator picks the outcome the instant the spin button is pressed; the reels only display a result that is already fixed.
A coin passed sensors that checked its size and weight, a lever or button released physical reels, and the stops on each reel decided the symbols.
Each spin on a Class II machine is really a round of bingo against other players, and the reels simply present that draw's result in a familiar form.
Autoplay runs a chosen number of spins automatically, and limits set on total losses or on a single large win halt it before the count runs out.