What kind of random numbers source does getnewaddress in Bitcoin Core API (bitcoind) uses? Is it an internal PRNG? Is it fed  from /dev/random or /dev/urandom? How secure is it? If I have a hardware (TRNG) plugged in feeding my /dev/random will this source of entropy get used by bitcoind?
As others mentioned Bitcoin core uses OpenSSL random sources.

This means that it uses any random source available, like:


the operation system, i.e. interrupts
random sources of the CPU or the chip set
dedicated hardware for entropy generation


So in order to make sure your hardware random generator works with Bitcoin you must make sure it works with OpenSSL.