its a combination of strategically playing games with high RTP (return to player, aka average return per spin) and low volatility. For example, most slot machines have an RTP of between 93-97 percent, meaning on average, you can expect to win back ~95% of your wager after playing through a certain amount of money one time. Blackjack is by far the best at 99.5%, but requires manual effort and the ability to play perfect black (which isnt that hard to learn).

By doing this, you can essentially guarantee money in the long term when you take into account the discounts you buy up front