I wish they would've interviewed Carol Anderson for this project. She wrote an excellent book on the topic titled "One Person, No Vote" which was published in 2016. She wrote about everything mentioned in this documentary and more.
"Rigged" points out the attempts of the Republican Party to suppress the vote of minorities who overwhelmingly vote Democrat. The tactics they use are slick and underhanded, but they all start with, first, exacerbating a problem that doesn't really exist: voter fraud. By pointing to the infinitesimally small amount of cases of voter fraud as a proof of something grander, they effectively garner the support needed to make very restrictive voter laws that disproportionately affect people of color and the poor. By using phrases like "we want to make voting easy and cheating hard," they can paint themselves as genuinely interested in honest and fair elections, but their track record points to anything but.
I think a documentary like this had to be made when you had the POTUS crying foul at every chance he got. The chant of rigged elections became so deafeningly loud that there had to be an equally loud rebuttal. "Rigged" doesn't nearly reach the decibel level, hence the ears, of as many people as Trump and his minions did, but it's at least an honest attempt.