I'm still hazy on how that went down.
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When they were in the stolen ship heading back, Poe told Finn over the communicator that Holdo planned on escaping the ship on the cloaked transports. The shot there is the camera is showing DJ's face and we hear what Poe is saying.
When they get caught, DJ traded the info for money/freedom.
As for the how, the cloak is really a power baffler that minimized power output so that you have to be close or do a strong scan right where the transport is or visually see it. Once the FO knew what was happening they did that and found the transports.
Just to cut off any comments about "How would we know the power baffling thing", the movie uses cloaked transports and decloaking scan. Perfectly understandable words for the movie.