How do we simulate randomness?

I came across this function randint() in Python that gives you a random integer from a list of integers. The thing that I am not able to digest is that how can we really simulate randomness. How can I really tell that a random function that may be in any programming language doesn't give a biased result? BellCurve?

How can we simulate something that is so natural as randomness? We can just calculate the probability of a result that can appear. But can never tell how this works.

For simulating something we need complete insight of the topic, don't we?