to the substance of what happened in that Special Grand Jury...witnesses with immunity are going to be key...from reports by noted attorneys, they typically have access to 'targets' and since they risk perjury charges, they'll be telling the truth to the real Grand Jury which does have the power to indict.
Link: https://www.marketwatch.com/story/foreperson-of-special-grand-jury-in-georgia-offers-unusual-glimpse-inside-trump-election-case-6374c800