The fundamental mechanism is still unknown, however we do know some important details about consciousness:

• It’s not a simple binary all-or-nothing
• It can change naturally or artificially
• It’s divisible and perhaps even additive

We know this due to a number of phenomena:

• Natural variation in states like awake, alert, groggy, asleep, comatose
• Altered states due to alcohol or drugs (drunk, high, caffeinated, hallucinating, suppressed with anaesthesia)
• Disorders such as Body Identity Dismorphic Disorder (BIID - thinking a major limb doesn’t belong to your body) or Phantom Limb (sensing an limb that isn’t there). Look these up if you’re unfamiliar, they’re fascinating.

Together these and other observations suggest that consciousness is an emergent phenomena (not present in simple organ structures alone) and occurs along a scale, likely proportional to brain size. And just as your daily state can change (between sleep and wakefulness at minimum) it seems a reasonable hypothesis that other creatures experience something similar, though perhaps with a lower maximum awareness in their most alert state.