Malware – How to attack victims from outside networks (WAN)

I started to be interested in security about a year ago, and of all that I have learned to this day, I never knew how to attack certain victims of outside network intruders, and I know that the victims are behind one Routers are located in a private network and can not be easily reached.

Now, I know Some victims are attacked by email, social media, or various forms of malicious content communication, and the attacker then receives a session by setting up port forwarding on their router so that the traffic goes directly to and on his or her computer I'm pretty clear. However, my question remains: how does someone attack a particular person on a wide area network (other than sending malicious content to open it) while hiding behind a router / firewall, or do they ever experience such attacks and attacks?

PS: Sorry for my inability to ask perfectly, but I hope you understand it. Thanks for that!