In a CoreXY setup both X and Y stepper move together. Technically the top left stepper is the X and the top right stepper is the Y

The right corner is assembled in virtually the same manner as the left. However I tried doing it in the same manner as the videos posted on the project page

Attach the top plate to the frame with 2x M6x10mm button head screws, 6x M5 T-nuts and 6x M5x8mm button head screws. Tighten the screws completely.

Insert 3x M3x25mm socket head screws into the stepper mounts. Slide 3x 19mm standoffs over the screws than thread the M3x25mm into the stepper motor but do not tighten.

The plug on the stepper motor should be facing towards the X motor