ESPN reads the letter about the Yankees sign stealing so you don’t have to:

The Yankees are alleged to have used the video feed that was originally installed for instant replay review to try to decipher pitch-signal sequences of opposing catchers, ferrying that information to baserunners who might have an opportunity to interpret signs for hitters. This is similar to the way teams have stolen signs for decades, with the added element of technology. Using the replay room to do so was common in baseball at the time the Yankees were reportedly doing it, according to players and staff at multiple clubs. The letter, from MLB commissioner Rob Manfred to Yankees GM Brian Cashman, did not suggest that the team used the video feed in the way that the Astros were later found to have done — immediate, real-time conveyance of pitch sign information to the hitter, through the use of trash-can banging.

ESPN.com

It was more sign learning than stealing.