Subjective consciousness is defined by the experience of qualia, one of which is a sense of a persistent and separate self-object.

We know what that means to us but we can only assume others have similar experiences.

And the only way to do that is by looking at behaviours.

So we point at cats and humans and say they're conscious because they demonstrate behaviours that we associate with our subjective experience of consciousness - goal seeking and planning, apparent emotional responses, awareness of context, decision making, pattern- and repetition-based learning, and other kinds of responsiveness and agency.

It's a very specific idea of what consciousness is, and it only really applies at human scale.

If an object/entity/subsystem experienced qualia of any kind - including the experience of any kind of self-object, from the most rudimentary to the most universal - without demonstrating any of the behaviours at a human scale, we'd never know.