Newbie: connecting (by scp) two remote computers

Wait, I used LOCALHOST and it doesn’t work. that means It doesn’t leave my computer. the router has nothing to do with this !
ssh localhost works, ssh localhost -p 4000 doesn’t,
and NO ssh [my public IP] works.

I don’t know whether my router blocks that port or not, nor how port forwarding works, nor if that’s what’s required ! For f***'s sake it’s so complicated, just for connecting two damn computers

I don’t know what you expect when you are talking about networking. Networking is extensive and in depth. Unless you have deeper knowledge and skills in this area it is just grasping at straws.

Well I need to know enough to establish an scp/ssh connection with my friend, and vice versa, and put my website online (that is another story though).