Is OnlineSubSystem Steam able to resolve the port problem ?

Hi , as far as I know , most of the router now block their port by default ,
If I have to directly connect to another game , the host has to enable DMZ or Port 7777 .

but I’ve tried using OnlineSubSystem Null , and it went well , I don’t have to setup anything on my router to succeed ,
but then I notice that OnlineSubSystem Null only function on LAN , and LAN is not block by router by default .

So I’m curious , here’s my question ,

  • how does some games on steam being able to play with each others without router blocking it ?
    (like those games , like Totalwar , Stronghold 2 , Magika , I’m pretty sure they’re host by one player and join by others)

  • If I use Steam SubSystem eventually , does that mean I have nothing to worry about the port problem ?

  • Any solution or tips for this ?

thanks for your time .