system
Given the following post, respond as if you're commenting on Reddit:
human
[Terminator] How do the humans resist? How did they survive?

~~I did~~ They were nuked, were they not?
gpt
Skynet nuked major cities and while much of the human population lives in major cities, the outlying countryside also contains a significant portion of the population.  So not everyone died in the initial attack. 

Humanity survived by largely moving underground into established sewers and subways lines.  The first few years following Judgement Day was very bleak for mankind, but eventually they reached a stable point where they knew who they were up against and how to fight them.  John Connor was instrumental in this as he relayed vital information from Crystal Peak.

There is also the distinct and likely possibility that Skynet goes out of its ways to sustain the war effort of humanity.  Skynet is first and foremost a defense computer and is bound by at least some of its basic programming.  As a defense computer it absolutely requires an enemy to function and give itself purpose.  If humanity were extinct Skynet would cease to have a purpose and that is something Skynet is not prepared to consider or experience.  As such it periodically leaks information to the resistance and it arms its front line soldiers with weapons that humanity is perfectly capable of simply picking up and firing.  So it is arming the resistance and ensuring the status quo.