[nbm]

TCP has an explicit client and server (some caveats here). The userspace API for TCP requires you to do a connect, which does the TCP handshake in the TCP stack, before you can send traffic.

For UDP, you just specify the destination IP and port, and send packets.

For TCP, new incoming SYNs to most (S)NAT addresses will just get dropped - especially CGNAT - making it impossible to communicate in that direction. If you're both in that situation (really common, actually), you just can't talk to each other.

For UDP, the packets will also get dropped on the receiver side. However, the act of sending the packet will often cause the originator's side's NAT to register that five-tuple (source and destination IP and port + UDP proto), which would allow the other side to reply. If both sides do this with the same IP/port pairs, then magically they can communicate - some of the time, at least. There's a lot more involved (for example, how do you know your own external IP address behind NAT?) - read up on STUN - https://en.wikipedia.org/wiki/STUN - for more details.