What is the mechanism that makes subjectivity exist?
Oh boy... This question is the exact reason I went into the field. I have no idea, but I hope we get the answer before I die.
At this point we are still just poking around in a vast mess of goo hoping to find some neural correlates of perception or motor planning or consciousness. Giving a mechanistic explanation for how these activity patterns produce subjective experience is going to take massive efforts, and maybe a leap of 'faith'. Probably the most progress will be made in areas of sensory perception or motor control, where we can be precisely quantitative about either the sensory input or motor output. Then we can record activity within various areas of the circuits responsible for a given modality, do high resolution reconstruction of the structure and connections between cells in that circuit, and then perturb the circuit, buy selectively silencing or activating functional groups of neurons on a trial by trial basis to probe how changing the circuit modifies the perception. Add to this the challenge of having to 'read-out' these effects via the behavior of an animal, rather than what a human can describe (who wants to volunteer to have channelrhodopsin virus injected into their head and a fiberoptic laser guide installed through their skull??). I don't think I'll work myself out of a job any time soon.
"The fundamental principle of science, the definition almost, is this: the sole test of the validity of any idea is experiment." -- Richard P. Feynman