Implementing the logic of creating "truly" random numbers may be hard which will generate truly random number with in a given range. But these logic were all implemented in every programming language and all you will need is to call the standard SDK to get a random number. Also RNG is part of every single card game. There can never be a card game without any RNG involved and draw RNG is the basic of all card games.RNG stands for Random Number Generator. It is like a computerized dice roll. Actually generating truly random numbers is something that is not as easy to achieve as you may first think. Ever had one of those CD changers that "randomly" plays the same CD or track over and over?
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But the current problem of Gwent is, the cards are so horribly polarized (top deck cards generating so much value than the low end cards, out of proportion) that missing a few (or in some cases even one card) determines whether you are going to win the game or not. If the cards are not this horribly polarized (4P cards get at least 6 points and 7 in some cases and cards like scenarios which can generate 18 or 19 [just by themselves and on top of that putting an engine which can generate even more points] for 13-15P. Now if you don't draw this card, you are losing HUGE point swing. Now you have drawn them and now the next deciding factor of whether the opponent can win or not depends on whether he had drawn Heatwave) the draw RNG will not play such a major role in whether one can win or not if not for the polarization.
Magpie brought a deck with twelve 4P cards and thirteen high provision cards because of this huge freaking polarization and the outcome of the game is mostly decided by whether he draws his top end cards or not. At the highest level of the tournaments, there is a bit of player agency involved, but at most levels, it is "whether I drew my cards and whether my opponents didn't draw the counter" determines the outcome. The game plays by itself in most parts.
This is what people tell when they mention RNG determines the game. Mostly meaning that the individual player's decision has little to do with the outcome (it is there.. the only thing now the players need to know is when to pass in R1) and the rest is completely determined by "who drew better" and the reason is bloody polarization.