If you mean the Presidium, the arms close around it.

Because there’s no Entrance

There are extensions on the front (rectangle) and back (conic?) of the wards (arms), that complete the closing sequence of the Citadel. Both can be seen in action in ME3, partially in the cutscene after Cerberus HQ and Return to the Citadel, after the final encounter with TIM.

The moments are brief, but visual.

If they could still enter thr citadel when its closed if there was still an opening that would make closing and sealing the citadel pointless which is why they have the smaller segments for the ends to fully seal the citadel

Here is a rear photo

Thanks @Thehumaneldar

Maybe something unseen covers it? Like a camera shutter made of super sci fi space metal or some sort of energy barrier? Otherwise, yeah, that is a truly gigantic hole in the rear of the space station, which is not at all affected by the closing arms of the feast of the station.

^nope. See the rear photo. There are panels that open and close to cove the back if the station is to get sealed