Everything about the remote bodies. Are they people who've had their brains replaced or wiped? Are they grown in a vat to be used as brute labor? Are they even human?! It's probably best not to think too hard about them...
Whatever the state of humanity is in this game, they seem to be teetering on the brink of something. Most people wear bright white masks (if they even have faces at all) and have no compunction against the aforementioned remote bodies being used, and they have no problem with hunting down a small child and shooting them with tasers or tranquilizers the size of said child's forearm. It seems to be a dystopian society built upon the ruins of a massive facility that has fallen into an extreme state of disrepair. If they don't care about that, then what do they care about?
I was wondering on my first playthrough if the artificial mind-control might somehow be connected to the parasites like the one controlling the pig that attacks you. Then I realized something: After you remove the parasite, the pig slumps over just like a remote body that isn't being controlled.
While the title is likely referring to the fact that you're trapped inside this gigantic dystopian factory, your perception of the word may switch once you're trapped inside the wriggling Body of Bodies.
"Inside" doesn't just refer to the facility. It's the boy's very goal: to get inside.
The majority of the "puzzles" in the game make more sense if you view them as being leftover examples of tests of the People Puppet technology throughout the facility that the boy has to co-opt to advance. Most of them likely served a specific scientific testing purpose years ago, but now they're just ways to circumvent obstacles in his path.
Why didn't the boy just go left? Even if he saw factories to the left and thought that going right would be a better idea, why didn't he go back out once he saw how deep he was going in? Unless you subscribe to the idea that the blob was controlling him.
Regardless of which particular theory you subscribe to, most people agree that the Boy was, for one reason or another, attempting to stop whatever was happening in the facility. This would require going into the facility itself. Plus, the path to the left was blocked off by sheer cliffs.
