Placing a bet on the live roulette table is simple: all you need to do is pick your desired amount (as long as it's higher than the minimum bets for that lobby), decide which bet you want to make, and click on it. From there, the live dealer will acknowledge your bet and place physical chips on the table.
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.