I am trying to understand how a cryptographic library works (for example, one that provides assymetric encryption such as RSA), but I'm running into a few problems about the key-generation. There are a lot of questions regarding the generation of random primes, but I'm interested in the generation of a random number. There seem to be some official guidelines by the government (for example, NIST FIPS 186-4 (PDF) is referred to in a couple of other answers to questions about random prime generation) for the generation of keys for certain protocols, but they are not an easy read and I don't think they answer my questions.
I think I understand the basic principles: Entropy is gathered by the operating system or a daemon to provided a small amount of unpredictable data that is used as a seed for a cryptographically secure pseudo-random number generator (CSPRNG). The CSPRNG generates the random numbers that are used to generate random primes (either a probable or provable prime).
What is a good CSPRNG? I couldn't find the algorithm that OpenSSL uses (it's very well possible that there are multiple). Wikipedia lists a bunch, but doesn't provide sufficient information on implementation and performance.
Also, are there any additional constraints on the prime or prime pair that is generated? I vaguely remember reading something about prime products of a certain form that are no good for use in cryptographic keys (because they are easy to factor with some tricks).