Generating random bits using two oscillators and a D Flip-Flop
This circuit will produce bits that are best described as not 'completely deterministic', rather than random.
The 'better' the oscillators, the more deterministic the result will be. If you had two 10 MHz high quality reference oscillators, then the output of your d-flop would be a long stream of one bit, flipping to the other when there was enough phase drift between them.
Running the sample oscillator at a tiny fraction of the frequency of the other enhances the apparent non-determinism, by ignoring a large number of the identical bits between changes.
Using 'bad' oscillators, like RC schmidt oscillators or IC ring oscillators for instance, increases the non-determinism yet more.
For a large difference in frequency, and 'bad' oscillators, the output will be random enough for many applications. Be aware though that any unintentional coupling between the oscillators, or change of a noise source like a power supply being replaced with a better one, may make the bits less random than expected.
A scheme like this would never be good enough for cryptographic purposes, or for running Monte Carlo simulations, better solutions exist for both of those.