TruetheVote obtained commercially available cell phone tracking data. That phone you carry around with you bounces off all the towers and geo-locates you as you move around. It can be purchased.
They then correlated that data with the video from the ballot drop boxes that was FOIA'd in Atlanta and elsewhere.
Eventually they had someone come forward and admit to the scheme and providing background on how it operated. And the video in the trailer corroborates the person's account of the activity/scheme.

It's detailed in this article. The John Doe stated $10 per ballot and he raked in $45k.

LINK
/georgia-opens-investigation-possible-illegal-ballot-harvesting-2020

The group also said in its complaint it temporarily secured the cooperation of one person, identified in the complaint as John Doe, who admitted he participated in extensive ballot harvesting during the November 2020 election and Jan. 5 runoffs and was paid $10 for each ballot he collected from a voter and delivered to a box. The man did not appear to understand his harvesting activities were illegal, but his participation was verified by his repeated appearance on the surveillance tapes, the group said.

"John Doe described a network of non-governmental organizations (NGOs) that worked together to facilitate a ballot trafficking scheme in Georgia," the group said in its complaint.

"John Doe's assignment included collecting ballots, both from voters in targeted neighborhoods and from NGOs that had their own ballot collection processes, delivering those ballots to other NGOs, picking up designated ballot bundles from the same group of NGOs, and depositing ballots into drop boxes spanning six counties in the metro Atlanta area," the group added.