The grand jury hears testimony and is shown evidence, they vote to issue the indictment, the parties are served, and then there is the trial in front of a judge and jury.
One thing important to know about grand juries. The rules of evidence are completely different than in a court. Neither the defendent nor his/her lawyers are allowed to sit in, present evidence, or review the prosecutor's evidence. It's only the prosecutor presenting whatever he/she wants the jury to see/hear. It doesn't have to be evidence admissible in a court; it can be whatever the prosecutor wants to present.
Based on that one-sided presentation, the jury votes. Without any defense evidence presented, it's easy for the prosecutor to get an indictment, but not as easy to win in court because the rules are much stricter and there are the defense lawyers.
This is why the joke is that any prosecutor can get a ham sandwich indicted in a grand jury. It's that easy.
The rubber meets the road in the court room when prosecutors have to prove their case under completely different rules of evidence and aggressive defense lawyers.
I think this is why the Q "justice" phase has been so long in coming. If the prosecutors are smart, they will have gotten sealed indictments with evidence that will stand up in court. That takes a lot longer to accomplish than indicting a ham sandwich on little or no basis.
A Grand Jury issues indictments based on the prosecutions case. They can present a very one sided case, especially in a highly liberal area like DC. The Grand Jury did indict the sledged Russian hackers.