I mean you can get dice weighted differently so you almost always get a 6 and a 1 for example.

There’s no point in weighing one die. The odds are the same as not weighing any die

Don't worry, there are dice in (at least some) prisons, every cell block had at least one D&D group going with 4-6 people, and they used dice.

Source: Was a prison guard in the ARMY for 4 years at Fort Lewis.

You had me in the first half for real

Just want to expand on this as it's the most direct explanation.

With two die there are 6 ways to you can roll a seven (each side has one way to add up to seven), and 36 total combinations (6 sides * 6 sides). So the odds are 6 times out of 36 or 6/36.

With one weighted die, you have a set value (say 3 for example). There is only one side on the other die that will equal 7 (4 in our example). So you have 1 out of 6 possibilities, or 1/6 chance.

However, this is only true for 7. If you were targeting 2 for example, the odds can change substantially. Normally you have one way to get 2 (1 and 1) so you'd have 1 out of 36 possible rolls or 1/36. If the weighted die was weighted to 6 though, you'd never be able to get 2, so your odds would be 0.

If a die is weighted, the first roll is no longer 1/6 probability to get a 7

Yes, actually, it is. No matter what the first die lands on, there is a 1 in 6 chance that the second die will land on the corresponding value necessary for a "7". You could glue the first die to the table with "6" (or any other number) showing, and there will be a 1 in 6 chance that the second die will bring the sum to 7.

Weighting one die (to favor "6") will increase the probability of every outcome over 7, and will decrease the probability of every outcome under 7, but the probability of rolling a 7 will not change.

You need to roll two dice to get a sum of seven. Consider two fair dice: No matter what the first dice lands on, there's a 1/6 probability that the second dice lands on the number you need to get a total of seven.

Consider now that one dice is weighted such that it always lands on 6. After you've thrown this dice, you throw the second dice, which has a 1/6 chance of landing on 1, so the probability of getting seven is still 1/6.

Of course, the order of the dice being thrown is irrelevant, and the same argument holds no matter how the first dice is weighted. Essentially, the probability of getting seven total is unaffected by the "first" dice, so it's 1/6 no matter what.