To add to the other comment a Grand Jury is also often made up of jurors called in using the prosecutor's private phone contacts. A few ex-cops and former work buddies. People also go to jail because of misuse of power by grand juries. There's a great documentary on Netflix but I can't remember its name right now. Suffice to say there aren't grand juries in any well working and fair justice system. It's abuse and/or theater 100% of the time.
I will offer a counter-example to this. I was an alternate for my local county's grand jury for a year. I was selected through the same voir dire process used to place me on a petit jury for a criminal trial years later. It was wholly random. AFAIK, I didn't end up in the jury pool because I knew a prosecutor or cop. Were "I know a guy who knows a guy" the selection criteria, I would have never ended up on the grand jury, as my father was personal friends with a local defense attorney; the question posed to me wouldn't have been if I hate cops, but whether I knew or was associated with anyone sharing the same name as my father.
The main difference I assume is that for a normal jury the defendant can challenge jurors, while for a grand jury the defendant often doesn't even know it is sitting on their case.
I could be wrong but my understanding is that grand jury is sort of like a trial jury (trail as in demo/mock, not a court trial) to show the evidence and case from the prosecutor side only in order to get an indictment. You can perform a grand jury multiple times in order to get the outcome indictment you need to finally charge someone. If you can’t convince a grand jury to get an indictment, then you will have an even harder time when the case goes to actual court with the defence being present with their own side of story.
So I guess there’s pros and cons to this. Ultimately the actual court trial is what matters but of course an indictment is mostly enough to destroy someone’s reputation even if they get acquitted later on (I think government has a 95%+ success rate or something).
This is not accurate in the US. I was called in via the same system as the petit juries- my name was selected from voter rolls and I received a summons in the mail. Law enforcement and criminal law professionals are specifically filtered from the process.
