They do whatever the law empowers them to do, and any state or city could empower "juries" to make laws. And there are countries where "juries" do make laws, though maybe they're not called "juries" -- but what they're called is just semantics.
They are instructed in the law by a Judge.
They can be instructed by whoever is needed to instruct them in their function. A quickie training course in legislation could be required, after which the "jurors" could perform the legislation function just as well as the preacher-speechmaker-demagogue-blowhards you like having do that function. But you can't give any reason why only your demagogue speech-makers have that capability.
The question is why it couldn't be done differently than this. Why must we rely only on these demagogue speech-makers to do the decision-making?
The jury's duty is to examine the evidence presented and render a verdict based on the instructions provided by the Judge.
Their duty is whatever the law says their duty is, and the law can be changed to empower them to do much more than only render verdicts in trials. Grand juries do more than what you're talking about. There are different kinds of juries, and different rules for them according to what jurisdiction it is. Not all "juries" fit one mold, as you're describing.
And they can receive their "instructions" however it is practical, not necessarily from a professional who requires 10 years of higher education. It's not true that we must have a system run only by elitists paid $300 - $400 thousand per year, thus causing the system to get so clogged that the needs and issues are delayed for years in order to be resolved.
It's not true that ordinary citizens are good for nothing other than to be put into factories to do makework jobs created for them by Bernie Sanders and Donald Trump to keep them out of mischief. Ordinary people can make decisions, contrary to your elitist philosophy that only demagogue professionals have any ability to think.
The decision rendered by the jury can also be set aside by the trial Judge, or by other Judges during the appeal process.
Who can easily be bribed. And who are political appointees. And the truth is that they hardly ever do "set aside" the jury decision. But if you think they should, why not also have the Judge set aside the decision of the voters, if they vote for the wrong candidate? or set aside the elections entirely because if the voters are so stupid at making decisions themselves, they are equally stupid at choosing who should make the decisions?
If you hate ordinary citizens making decisions, then why let them have any decision-making power at all, about anything? Obviously you think they are ignorant rabble incapable of understanding anything, so what is the point of having them make decisions at all?
Why do you think ordinary citizens are incapable of deciding anything other than that of choosing which demagogue tyrant to rule over them?
The question is why ordinary citizens should not be given a greater degree of decision-making power than in the current system. It's about what the process should be, not what it currently is.
If you're right that only elitist speech-makers are capable of making decisions, then why not also have them render the verdicts instead leaving this to the "jury" which is chosen from among the rabble citizenry who are too ignorant to make decisions about anything important?