NAT Routers Part 1: Client Firewalls
Segment 7: Packets to and from the Internet
The router dutifully sends your packet on out to the Internet.
Typically, the TCP data will be sent to process 80 (the "well-known" port address of a web server) somewhere out on the Internet, which will eventually respond, sending back one or more packets in answer. When any of those answering packets returns from the Internet to your NAT router, the router will think:
...Ah! This is coming to my process 3000. I know where that goes!
...and it will relay the response back to your PC, so that it can speak with the web server, maintaining a 2-way dialog exactly as it should.