2/1 odds means you'll winR$2 for everyR$1 you bet. AR$100 bet at 2/1 odds would winR$200, for a total payout ofR$300.

For a game like roulette, the online casinos will have a live dealer (or sometimes just a live wheel where the ball gets inserted automatically). All your bets are placed on your phone screen but the wheel spins in real life, and then the result is seamlessly digitized, so you're paid out immediately.

While mathematics can help you understand the odds and probabilities of different outcomes in a roulette game, there is no guaranteed way to win using mathematics alone.