The most convincing theory to me is Integrated Information Theory.

Basically the more integrated the information in a system is, the more potential for conscious experience. Different structures or "shapes" of information translate to different qualia, like the color red or the sound of a note.

Integration in this case means that the whole is greater than the sum of it's parts; you can't remove a piece without affecting the whole/most of the system.

This is an answer to the Ship of Theseus problem, because the old pieces of wood are getting un-integrated from the whole, they are no longer the ship. The ship made from old pieces is a new ship, with it's pieces integrated into a new whole.

Animals would be as conscious as the complexity of their brains (not necessarily smarter), and yes this is a spectrum of consciousness.

Computers and AI are far off from this requirement. We purposely designed them to be modular and not overly integrated with itself. Even LLM's have their neurons in strict layers that has information travel from Input to Output.

No concrete proof, but the theory is very robust and could maybe even be provable one day.