Ok so this can quickly become very complicated.

Traditionally, the dominant religions tend to suggest a conscious entity called God imparted it into some clay with its innate magic.

Modern interpretation suggests that living things that can sense things in their surroundings will do better than ones that can't. If you can't see or hear a bear approaching, you are more likely to be eaten, and the same holds true for your own food, shelter, and baby-making.

All that sensation is useless without a response to what is causing the sensation. So the ability to move towards food or away from a threat is important too. So these faculties tend to start with simple rules and build in complexity over time, bit by bit, depending on how helpful it is to survival. Memory of locations helps too - you can find likely places food will exist, and find your nest after leaving it to find food for your babies.

Centralising and organising all these sensations and reactions so you can connect cause and effect is probably the start of consciousness. It will become an increasingly complex system of sensations, filtering and consequential behaviours within an expanding mental map of the surroundings built from the memory of sensations.

Having a mental resource/memory means you can learn what causes specific effects in your surroundings, including your own movements and decisions. This is the basic form of self awareness - me as an interior mind Vs the world. This is an advanced form of consciousness.

This can happen if you can sense things, respond to things, remember things. It can become more complex if your sensory system can detect activity in other parts of its own senses to moderate/inhibit/emphasise your responses. You don't want to listen to your blood and heart all the time, you don't want to spend all the effort on controlling your breathing unless you need to.

This sensation of sensation, and the inhibition of sensations, is the mix of consciousness and unconsciousness that makes everybody who they are.

There are known physical things, like punches to the heqd and anaesthesia, that can halt consciousness. You can't see without eyes and you can't think without a brain, which is why brain damage and dementia can make people confused and change personality. It's also why we can do surgery and people don't feel it.