It's most often neurological AFAIK, with some susceptibility to psychological and physiological modulation, like responding to blood flow modifications but also to where your attention is or how you're coping with it.
The most recent interesting thing for me has been the realization that it also has adaptive negative-feedback qualities, like many biological mechanisms, and responds in counterintuitive ways to some interventions: I used to think that because many city noises trigger it and make it seem immediately louder, uncontrolled exposure to city noises was always a bad thing, and the more I could isolate myself from that, the better off I would be.
Not so. Too many consecutive days spent with too many hours of wearing earplugs
actually exacerbate the tinnitus (probably by raising my neurological sensitivity just like the initial hearing loss made the perceptual system overcompensate by generating too much fake signal at those frequencies), and "the cure" is to spend reasonably dosed amounts of time
intentionally exposed to regular non-extreme everyday noises of all kinds, and
then the tinnitus subsides to my long-term-average, easily ignored level.
