I don't know what you expect precisely when you ask for "detailed case-studies", but if PC-B has no Internet access, it is a well-known practice for the attacker to use PC-A as a relay between PC-B and the C&C:
PC-B <---> PC-A <---> C&C
To give a more concrete example:
- PC-A belongs to the users LAN, this is the computer where people access to their emails and browse the Web. Sadly, one email or web page was infected, thus contaminating PC-A.
- Once in running in the LAN, either in an autonomous way or remotely controlled by the C&C, the virus starts to scan the network and try to propagate itself. It reaches PC-B (unpatched vulnerability, poor password or whatever) and contaminates it.
- PC-B belongs to another, restricted network with no access to the Internet (with chances that it has access to more interesting resources!). The virus can setup a way for PC-B to use PC-A as a relay to get new orders from the C&C and exfiltrate data.
This relay can take any form depending on the attacker fantasy and requirements. It can range from simple proxy-like features to a full fledged peer-to-peer network ensuring that the virus' network will remain functional even if some relay machines are being decontaminated.