It's all in the game, like Rainmaker said. The house has an advantage without needing to screw with the RNG. It's all about the payback odds. No RNG should ever be programmed. It doesn't know what it's making random numbers for. It just picks a random series of numbers.
Take a really simple example and you'll see how it works. Let's say there's an online wheel of fortune with 10 possible spots it can land on. None of those spots pay anything except for one, and that spot pays 9:1. There's no house edge in this game because every time you spin the wheel you lose $1, and one out of ten times you get your $1 back plus $9. The way casinos make money is whenever there's a situation like this, they change the spot to pay 8:1. That would give the casino an edge of 10%.
When it comes time to spin the wheel, the wheel program just says to the RNG, give me a random number. The RNG is constantly running through thousands of numbers. It says, okay, here, and it spits out a number between let's say 1 and 16 million. Let's say that number is 245,493. The wheel program then goes 245493%10...meaning it realizes 245,493 is too high a number so it divides by the highest number it's looking for (10) and takes the remainder. The remainder is 3 in this case. So it spins the wheel the the 3rd position, and then figures out if that position is the winner.
A casino can rig a game, but it can't really rig an RNG itself. Take that situation. The RNG doesn't know what it spit a number out for, it doesn't know how many digits the wheel is looking for, it doesn't know what each position would pay. The RNG is basically just a dumb thing that's constantly running through random numbers and spitting them out for games. If a casino wanted to rig the game I'm talking about, what they'd do is this:
Wheel asks the RNG for a number. RNG says 3. Wheel says, okay, 3. Then the wheel runs a piece of bad code some casino guy put in there and says, oh shit, 3 is a winning number. Pick again. RNG says, 3, again, randomly. The wheel says okay, we tried once, this time we let it go through and the guy wins. It's called double dealing.
But the casino doesn't need to rig the game either. They just need to set the odds to 8:1 on that winning spin, and they're going to make a solid 10% over time.
Hope this kinda explains things, if you got more questions about the actual programming of these things I'd be glad to tell you as much as I know about it.
).
It was a bit clumsy written from my side. 
