How is the RNG for games/gambling developed? Is it really random?

Is it based solely on probability or are there other factors involved? Are there certain rules that need to be followed for gambling RNG? Do games use the same rules?

Players in games with the same stats/gear/health battle, and it is up the the RNG who wins, allowing players to gamble in settings where developers may not have intended that to happen.

Is RNG really that random? I know that C++ rand() is dependent on time and will give the same number unless you seed the generator. It seems some people can use RNG to their favor in games/gambling settings.