From what I can gather.

-Holston is dead. The helmets are modified to play a video to trick them into taking off their helmet. What you see on the monitors in the silo is real. The air is bad.

-Juliet noticed the video in the helmet by the birds flying at the same time from the video she saw on the hard drive. Confirmed it was a fake by putting the badge on holston through the projected/illusion of a rock there.

-The suits were designed to fail. The tape they use was trash and they knew it in supply, who had better tape, which appears to be what makes the suit air tight. Supply slipped her the word that they were swapping the tape on her suit. Which is why Juliet lived and put 2:2 together with, “supply knows best “ or whatever.

I'm willing to bet that the big door at the bottom is access to the other silos.

I'm also quite sure that Holston is probably dead. Along with everyone else, that went outside.

I had this thought last night, after seeing the finale. What if the other silos are open to each other, and only Silo 18 (I'm guessing it the number based on the key to the mainframe having number 18 on it) is closed off / isolated? Maybe because of it being the origin of the rebellion? As some sort of sanction. Or maybe to contain The Syndrome - whatever that even is.

I know it's far fetched. But who knows? (Other than the millions of people who've read the books, of cause ;-))

1. It’s his body. The tape is the answer. When making everybody’s suits they used IT tape to seal the suit. That tape let in radiation or whatever and killed people sent to clean. In Julie’s case they used superior mechanics tape that properly sealed her suit allowing her to survive
2. I would wager entrance to other silos