In the theatrical cut of T2, John asks the T-800 whether it can learn stuff that it hasn't been programmed with, so that it can become more human. The T-800 then explains that its CPU is a learning computer, and that the more contact it has with humans, the more it learns.
JOHN: Can you learn stuff that you haven't been programmed with? So you can be... you know, more human? And not such a dork all the time?
T-800: My CPU is a neural-net processor; a learning computer. The more contact I have with humans, the more I learn.
JOHN: Cool.
Terminator 2: Judgment Day (1991) - Theatrical Cut
The dialogue in the Special Edition of T2 is a bit different. According to that version of the film, when Skynet sends Terminators out alone, it presets the switch on their CPUs to read-only, which apparently limits their ability to learn and grow beyond their original programming. However, John and Sarah reset the switch after that, in order to restore that ability.
JOHN: Can you learn stuff that you haven't been programmed with? So you can be... you know, more human? And not such a dork all the time?
T-800: My CPU is a neural-net processor; a learning computer. But Skynet presets the switch to read-only when we are sent out alone.
SARAH: Doesn't want you to do too much thinking, huh?
T-800: No.
JOHN: Can we reset the switch?
Terminator 2: Judgment Day (1991) - Special Edition
In both versions of the film, the T-800 initially shows no signs of human emotion or empathy, but gradually does so more and more as the film progresses.
In Terminator: Dark Fate, we see a T-800 -- now going by the name of 'Carl' -- which completed its mission to terminate John Connor back in the '90s, and has subsequently spent the last twenty-plus years living amongst humans, with no further instructions from Skynet as to what it should do with itself.
Dark Fate appears to treat the theatrical cut of T2 (rather than the Special Edition) as canon, because Carl evidently learned to become more and more humanlike over time, without John or Sarah (or any other human that we know of) tampering with its CPU.
According to Carl, all that time living amongst humans -- and in particular, the experience of raising an adopted son -- enabled it to develop the equivalent of a conscience.
SARAH: Nice family. She a Terminator, too? That's your little Terminator kid?
CARL: His name is Mateo. I met his mother, Alicia, a few months after I killed John.
SARAH: Oh, you don't get to say his name. Ever.
CARL: Her husband had beaten her. He was trying to kill her child. She had nowhere to go. Caring for this family gave me purpose. 'Cause without purpose, we are nothing.
SARAH: Touching story. Does it have a point?
CARL: While raising Mateo, my son, I began to understand what I had taken from you.
GRACE: Wait. You grew a conscience?
CARL: The equivalent of one, yes.
SARAH: It's an infiltrator. It's lying.
CARL: When my mission was completed, there were no further orders. So for 20 years, I kept learning how to become more human.
Terminator: Dark Fate (2019)
If we go by the films alone, it's unclear as to precisely why T-800s have this capacity to not only learn more and more about humans by interacting with them, but to gradually become more humanlike, to the point of developing empathy with humans, and the equivalent of a conscience.
During interviews conducted to promote Dark Fate though, James Cameron explained that T-800s are actually programmed with the goal of becoming more human, so that they can blend in with humans and avoid a scenario where human society ends up hunting them down and destroying them.
You could potentially pick holes in the logic of what he says here, but that's the clearest explanation we've been given as to why T-800s become more and more humanlike whilst interacting with humans.
JAMES CAMERON: I had some ideas for Terminator sequels that had lain dormant for a long time. But I knew that, of course, Arnold as an actor, as a human being, was getting older, and so the later you come back to that -- with him in the role -- you have to deal with it. But I thought, "Let's make that an asset." Alright, so we're going to cut to a Terminator that was sent back in time, to some time in the '80s or '90s, hit his target, and had no further purpose. And was waiting for orders from a future that had ceased to exist. Because when Sarah changed Judgment Day, and her actions led to the demise of Skynet -- or the non-emergence of Skynet, really; it never existed in the first place -- the Terminator had no boss. He's got a learning computer; his central processor is a neural-net chip, it's a learning computer. He's given a mission, not only to execute a specific mission, but to blend into human society so he can operate, so he doesn't wind up with all the forces of human society against him, hunting him down and destroying him. So he can just kind of blend in and be an infiltrator, that's the concept, right? What happens if you leave that learning computer switched on, learning, with the goal, the programmed goal of becoming human, for 25 years? We saw him in movie two, over the space of two or three days, already starting to change, already starting to become, you know, warmer, funnier, use language better, right? What happens in 25 years? So, that was, I think that was the challenge that I brought to the writer's room. I said, "Let's do this. This is gonna be radical. This is gonna be cool. We're gonna create a Terminator who wants to be human."
TERMINATOR DARK FATE Full Cast Interviews
