Zack Snyder's Justice League may hint at how Cyborg learns to build the Knightmare timeline's cosmic treadmill. Although HBO Max's 4-hour Snyder Cut concludes with the Justice League defeating Steppenwolf in conclusive fashion, there's clearly a bigger story at play, with the victorious heroes getting a glimpse of the DCEU's big bad, Darkseid. There's also an epilogue set in a post-apocalyptic future where Darkseid reigns supreme and Superman serves at his side, having been corrupted following the death of Lois Lane. In this grim reality, only Batman, Cyborg and Flash remain of the original band, and they've recruited the motley crew of Joker, Deathstroke and Mera towards some unknown purpose.
Based on what Zack Snyder has revealed about his abandoned future Justice League movies, Bruce's B-team would've stolen a Mother Box with the help of Joker, then constructed an other-worldly DC time-traveling device known as a cosmic treadmill inside the Batcave. Using this contraption, Flash is able to travel back through time and warn Bruce of the dangers to come, resulting in Ezra Miller's "dream" cameo in Batman v Superman: Dawn of Justice. One would assume that 2 films spent fiddling with history means the Justice League successfully undo Darkseid's invasion. Thanks to the events of Zack Snyder's Justice League, the heroes obviously know all about the Mother Boxes' potential. Less clear is how they learn about the cosmic treadmill - hardly something Barry would've picked up in science class.
An answer may be found in Superman's resurrection scene. Zack Snyder's Justice League establishes that Cyborg was built by his father using Mother Box technology and, as such, holds an intrinsic connection with the three devices. Zack Snyder has also confirmed that the Knightmare timeline's cosmic treadmill would've been a product of the Mother Boxes - presumably the one Joker helps procure. Shortly before bringing Superman back to life, Cyborg experiences a vision of the post-apocalyptic future, similar to the flashes Batman has been experiencing. This reverie seems to have been triggered by contact with a Mother Box, so it stands to reason that Cyborg also could've seen how to harness the box's power to make a cosmic treadmill. The very same vision might've also revealed to Cyborg that Barry Allen can use the treadmill to rewrite history, rather than just rewinding a few minutes to fix a blunder.
Bruce, Victor and Barry are three of DC's sharpest scientific and engineering minds, but surely even they wouldn't have thought to create a time-bending treadmill for Flash to go running on. As the only League member with a direct connection to the Mother Boxes, Cyborg is the most probable source of inspiration for the plan. He alone would know how to manipulate that alien power to create a machine far beyond humanity's capability. In the Zack Snyder's Justice League Knightmare, Cyborg is also more comfortable with his mechanical body, suggesting that he continues to uncover the secrets of the Mother Boxes as time passes. Victor might even peer into the work of Silas, his father, to gain even greater insight into the limitless possibilities of Mother Box building blocks.
The creation of the cosmic treadmill, whether by Cyborg or through other means, is just one of many untold stories hiding in the background of Zack Snyder's Justice League. It seems ridiculous to describe a 4-hour movie as "the tip of the iceberg" but that's certainly how it feels watching Darkseid, Knightmare, and time travel sprinkled throughout the Snyder Cut. The possibilities of the cosmic treadmill are especially enticing, and represent a natural extension of the upcoming 2022 The Flash movie, which incorporates the Flashpoint comic arc and DC's multiverse.