I have always wondered what can affect the RNG and generating keys with enough entropy? For instance, do we know which hardware and software are always good sources of entropy and which aren't?
If only it was that simple. If you make a quick search, you will notice that even from experts in the field, there's a moment of doubt when it comes to verifying that the RNG is true. Intel and AMD chips come with an RNG that is impossible to verify completely as far as I'm concerned. You cannot distinguish a pseudo-RNG from a true RNG, because you cannot detect if there is a function that deems deterministic the number generation.
When it comes to software, /dev/urandom and dev/random are good sources, as for hardware
there has been development, but if you merely want to run a Bitcoin wallet in an air-gapped device, then you don't need hardware for that purpose. Just roll a dice or flip a coin. It is trivial and completely verifiable.