Sure, but by that (correct) reasoning the fact that it’s SMS (as a system) or some other system is similarly irrelevant to them and not the terms they think in.
Answer: "idk, tech magic." They know there is some way that isn't actually magic, they know there are mechanics, but they don't know or need to know what the mechanics are.
How it knows to display a green bubble or a blue bubble is exactly like that. That's what I mean by it's not a remarkable thing to anyone that the txt app does this. Q: How? A: Somehow. They don't see it and think "Wow, this proves that there are two unrelated back-end systems."