The RNG, or Random Number Generator, is part of the game software. It’s the piece of code that guarantees the randomness of the results on virtual casino games.
RTP is a rate indicating the theoretical return to the player. It gives information on how frequently the game is likely to give winning results. However, if a game’s RTP is 99%, it doesn’t mean that for every 10th spin, the player is guaranteed to win since the results are random, and this is a percentage that is obtained on thousands of turns in the machine to give a probability.
The UKGC mandates independent third-party agencies to regularly control casinos and providers on the games they offer to the public. They control that what they promise the player is really there and that the games are fair. For a game to be fair, it needs to have random results and respect the probability rate guaranteed by the casino and the provider.