Question

For instance, could it be used to generate a one-time pad key?
Also, what are its sources and how could it be used to generate a random number between x and y?

Answer 1

The only thing in this universe that can be considered truly is one based on quantum effects. Common example is radioactive decay. For certain atoms you can be sure only about half-life, but you can’t be sure which nucleus will break up next.

About /dev/random – it depends on implementation. In Linux it uses as entropy sources:

The Linux kernel generates entropy
from keyboard timings, mouse
movements, and IDE timings and makes
the random character data available to
other operating system processes
through the special files /dev/random
and /dev/urandom.

Wiki

It means that it is better than algorithmic random generators, but it is not perfect as well. The entropy may not be distributed randomly and can be biased.

This was philosophy. Practice is that on Linux /dev/random is random enough for vast majority of tasks.

There are implementations of random generators that have more entropy sources, including noise on audio inputs, CPU temperature sensors etc. Anyway they are not true.

There is interesting site where you can get Genuine random numbers, generated by radioactive decay.