Looks like the Red Sox were stealing signs using Apple watches
The claim doesn't appear to be that the Red Sox used Apple watches to steal the signals (although that's what pretty much every headline says), but rather that they used video replay to steal the signals and then Apple watches to disseminate that information. The precise line and method of communication isn't entirely clear from any of the articles I have read, but the most direct reading suggests that the replay personnel communicated the information to one or more persons in the dugout, who viewed this information on Apple watches and then used hand signals to inform players at bat.
It would appear likely that the replay personnel simply used iMessage or text message to send the information and that these messages were simply viewed on the Apple watches, just as they could have been with any other mobile phone, tablet, or smart watch.
The headlines make it seem like there is something technologically advanced going on, when it would appear that the method is the exact same that has always been used to steal signals / calls, both in football and in baseball: People looking at video telling people on the sideline / dugout, who then inform relevant players on the field.