FTP is not the best example as it's behaviour change depending on Active/Passive mode.
Consider HTTP instead, which is simplier.
- Browser_A initiates a TCP connection to ServerIP:80
- PC assigns a random port, example HostIP:55123
- TCP Connextion between HostIP:55123 and ServerIP:80 is established
- Server receives a request and responds to HostIP:55123
- PC knows every TCP packet from ServerIP:80 to 55123 should be delivered to Browser_A