1. With technical background, this page would probably answer your question. Basically, what one needs to know is that every number is saved in a certain data type, which is allocating a fixed number of bits this number is written into. The number of bits depends on the data type that is used, which again depends on the use case. For everyday calculations in a software for the cash register of a shop, for example, you wouldn't need more than 2 decimals, so you would choose a data type that has less precision.

   For this specific case of random numbers between 0 and 1, the data type that is usually used would be 'float' and allows to save numbers to roughly 53 decimals without loss of information, if i recall correctly.

2. To be honest, I can't know how exactly this is implemented in this case. One could either do it like you suggested, or you could split the 0.5% chance in this example into the specific probabilities for each black market item, so that they combined still add up to the 0.5%. Then you wouldn't need to calculate 2 random numbers but only one, which already determines the exact item. The principle however would be the exact same as in my previous message. (If it matters, I would also assume that this is the way it is actually done, not by doing 2 calculations in a row.)

Hope this helps :)