The best answer I've seen to this is that it doesn't. Consciousness is a self-reported quality, which is to say that there's no way to tell if something is conscious without asking it. Furthermore, many things can answer 'yes, I'm conscious' even if our intuition says they aren't actually conscious (computer chatbots, choose-your-own-adventure books, post-it notes with 'Yes, I'm conscious' printed on them), so there's no way to tell for sure if something is conscious even if you ask it. The "Philosophical Zombies" thought experiment tells us that it's perfectly plausible to imagine a society where nobody is conscious but nothing is different.

What do you call a quality that can't be measured, detected, or tested for, and the absence of which causes no difference whatsoever? You call it 'imaginary'.

Consciousness is just an illusion born from our brain's predictive and empathetic abilities to simplify concepts of agency.