https://github.com/jedisct1/libsodium/blob/master/src/libsod...
They sum it up in their docs like this:
- On Windows systems, the RtlGenRandom() function is used
- On OpenBSD and Bitrig, the arc4random() function is used
- On recent Linux kernels, the getrandom system call is used
- On other Unices, the /dev/urandom device is used