It was a "service hatch"
WIDE SHOT as Rey holds on carefully as the guards walk off, not seeing
her over the edge, a death drop below her.
But then she SEES SOMETHING and has a brainstorm. She strenuously
cross-climbs to an invisible SERVICE HATCH, which She opens, and climbs INSIDE THE WALL, closing the hatch behind her.
Working her way sideways, hand over hand, while methodically locating
shallow footholds, she made it across to a service hatch slotted into
the wall of the atrium. It opened, silently and without the need to
enter a code, at her touch. While still yawning to depths unknown
beneath her, the inner workings of the base that she was able to reach
through the hatch provided access to better handholds. If she was
lucky and didn’t lose her sense of direction, much less her grip, she
felt she might be able to work her way across to the corresponding
service area that ran underneath the TIE fighter hangar, avoiding the
guards above. Then she would have to find a way to access the hangar
deck itself and without drawing any attention. Assuming she could do
so, she could try to steal a fighter.