Consciousness is what the body is doing to be viable.

Without it we can't walk, eat, reproduce, or do anything. I like to think cost viability reasons explain consciousness. I know people prefer metaphysical or quantum magic explanations, I prefer a prosaic one - cost. It's a mechanism to keep our costs offset by gains. Cost can also explain unity - we die as one organism, not each organ on its own.

The definition of consciousness is quite clear IMO: "it is something to be that thing."

What's not clear is what causes it and how to evaluate whether someone/something else "has" it.

That's very imprecise, and not at all convincing.

Eg.

1. The brain "does cognition".

Cognition <> consciousness.

2. Some philosophical theories have consciousness pre-figuring complex arrangements like the brain.

ie. The brain is not necessarily a pre-requisite for consciousness.