I have basic networking question.

Scenario : Two pc's are communicating on a wan. Both the pc's ate behind routers or modems.

My question : Both the pc's have public IP of each other. That public IP is most of the time is either of the router or of the modem. There can be more then one pc's behind those routers and modems. Then how the pc's are communicating. I can understand the packets can reach upto those routers or modems. But what after that. In the packet , destination IP is public IP. Then how the router or modem decides where to send the packet? Can anybody explain me this please. Thanks in advance.