> Also how exactly are jury trials superior to e.g. Magistrates Courts in the UK?

The purpose of the trial is to separate the innocent from the guilty, and there is intended to be a presumption of innocence. But because the prosecution has to prove their case beyond a reasonable doubt, they'll tend to only bring cases when there is a high probability of guilt -- a good thing -- so then let's say 90% of the defendants are probably guilty and 60% are guilty beyond a reasonable doubt.

A judge is going to become intimately familiar with that. ~90% of the defendants are actually guilty, so the judge develops the intuition that a new defendant is very likely guilty. That's a presumption of guilt. Soon even the innocent ones are getting convicted, when the whole point of the process was to prevent that.

A jury is a fresh set of eyes who look at the defendant as the only case they're going to be deciding for the foreseeable future and haven't been prejudiced by a parade of evildoers sitting in the same chair. It's also twelve separate people who each individually have to be convinced.