Just anecdotal from my own experience. I worked for a fed judge but worked in the same office as interns for a magistrate.
I think for both jobs, its pretty similar for work product. The main process is working on an order on the calendar for some type of motion. The clerks assign the intern a motion, the intern gets the attorney's briefs, writes a tentative order, edits a thousand times, watches the oral arguments, and then talks to the clerks and judges and adjusts from there to a final order.
The interns I knew for the magistrate mainly worked on things like social security claims, motions to compel or other discovery issues, and other pre-trial stuff. They also tended to deal with a lot of pro se litigants.
In comparison for the art III judges, we worked on more broad subject matter and trials and dispositive motions.
More particular to my own experience, we'd do projects for the judge if asked, watch trials (especially verdicts and voir dire), prepare for motions in limine, and other time fillers in between larger assignments. I also did a
habeas corpus 2255.
More generally In crim law, Magistrates can do arraignments, misdemeanor crimes, give certain types of warrants, and sometimes evidentiary motions. Largely things that will save a busy Art III judge some time.
For civil they do lots of non dispositive orders things like discovery motions. Or if the parties consent they can go through adjudication (I.E. the social security stuff I talked about above)