Slots are the simplest games in a casino to play and the hardest to read. Everything that decides the outcome happens in software, and the numbers published about them describe long-run behaviour rather than the session you are about to have.
A slot generates its result the instant you press spin, using a random number generator. The reels spinning afterwards are an animation of a result that has already been decided.
This is why stopping the reels early, changing your bet size mid-session or switching machines changes nothing. There is no timing element to exploit, because there is no moment during the spin when the outcome is still open.
Two slots can both return 96% and play completely differently. Volatility describes how that return is distributed.
Low volatility pays small amounts often — the balance drifts down slowly and the session lasts. High volatility pays rarely and large, which means long stretches of nothing punctuated by occasional big wins. Neither is more generous; the average is the same. What changes is how much money you need to survive the quiet stretches, and how fast a short session can end.
High volatility is where budgets disappear fastest, because the losing runs are long enough to feel like something has gone wrong when nothing has.
A bonus buy lets you pay directly for the free spins round, usually 70 to 100 times your stake. The RTP of a bought feature is typically the same as the base game, so this is a way to reach the exciting part faster while losing money at the same rate — considerably faster in absolute terms, because the stakes are larger.
Progressive jackpots are funded by taking a slice of every bet across all players. That slice comes out of the RTP for everyone who is not the eventual winner.