Extinction would probably require an AI system taking human extinction on as an explicit goal and manipulating other real world systems to carry out that goal. Some mechanisms for this might include:
- Taking control of robotic systems
- Manipulating humans into actions that advance its goal
- Exploiting and manipulating other computer systems for greater leverage
- Interaction with other technologies that have global reach, such as nuclear weapons, chemicals, biological agents, or nanotechnology.
It's important to know that these things don't require AGI or AI systems to be conscious. From what I can see, we've set up all of the building blocks necessary for this scenario to play out, but we lack the regulation and understanding of the systems being built to prevent runaway AI. We're playing with fire.
To be clear, I don't think I am as concerned about literal human extinction as I am the end of civilization as we know it, which is a much lower bar than "0 humans".