How does linux / netfilter / iptables "know" if a packet is meant for itself for to be forwarded

Question

I have setup an ubuntu box as a router manually , using iptables + NAT.

But I was wondering, how does the "middle-router" know if the packet is meant for it or to be forwarded?

I tested this scenario:

                SYN                                       SYN
                src:192.168.3.3.8080                      src:12.12.12.12:8080
+--------------+dst:5.5.5.5:443       +-----------------+ dst:5.5.5.5:443
|              |                      |   Linux Router  |
|   Client     +--------------------->|                 +------------------>
|              |                      |LAN: 192.168.3.1 |
| 192.168.3.3  |                      |                 |
|              |<---------------------+WAN: 12.12.12.12 |<------------------
+--------------+SYN-ACK               +-----------------+ SYN-ACK
                src:5.5.5.5:443                           src:5.5.5.5:443
                dst:192.168.3.3:8080      (HOW???)        dst:12.12.12.12:8080

                                      Webserver listening
                                      on 0.0.0.0:8080

And it works as expected on the client. But when the router receives the packet with DST 12.12.12.12:8080 (on which it is listening) , how does it know to forward it? I tried this manually with UDP as well (so no TCP states) and it seems to work.

Why is there no port conflict when its the same port?

My iptables NAT rule on the router:

-A POSTROUTING -o enp6s0 -j MASQUERADE